A Review of the Most Concerning Chemical Contaminants in Drinking Water for Human Health

Abstract

Chemical contaminants in drinking water, including arsenic, nitrate, and fluoride, pose significant health risks, particularly in low-income countries with inadequate water management infrastructure. This study aims to identify the most hazardous chemical contaminants, evaluate global drinking water quality, and assess health impacts based on a comprehensive literature review guided by the PRISMA method. The findings revealed that arsenic concentrations in Romania, Pakistan, and India exceed the WHO and USEPA safety thresholds, with maximum levels reaching 130.3 µg/L. Nitrate levels in India and Morocco were found to be as high as 844 mg/L and 270.1 mg/L, respectively, far surpassing safety standards. Fluoride contamination in Pakistan reached 30 mg/L, well above the recommended limits. These contaminants are primarily sourced from industrial effluents, agricultural runoff, and improper waste disposal. The study highlights significant regional disparities, with 67% of reports from low-income countries and 88% of contamination cases linked to groundwater sources. The results underscore the urgent need for improved monitoring, stricter regulations, and effective management strategies to mitigate health risks, particularly in vulnerable populations such as infants and children. Governments and international bodies must prioritise addressing chemical contamination to protect public health.

1. Introduction

Access to safe drinking water is recognized as a human right [1]. Despite global recognition of its importance and initiatives such as the United Nations’ sixth Sustainable Development Goal (SDG 6) [2], which aims to ensure the availability and sustainable management of water and sanitation for all [3], significant challenges remain. As of 2022, approximately 2.2 billion people still lack access to safely managed drinking water [4]. Achieving SDG 6 remains a distant goal, with considerable work needed to improve drinking water quality worldwide [5].

Drinking water can be contaminated by physical, biological, or chemical substances (see Figure 1) [6]. The primary focus of this paper is chemical contamination, which can have severe health consequences. Consuming water contaminated with chemicals significantly increases the risk of diseases such as cancer, hormonal disruptions, and cardiovascular issues [7]. Chemical contamination arises from both natural and human activities [7]. With the rise in industrial activities and urbanization, water sources are increasingly polluted, exacerbating health risks and highlighting the urgent need for effective management strategies [8].

Low-income countries are particularly vulnerable to the impacts of chemical contamination due to limited resources for laboratory testing and water quality management [9]. In these regions, the lack of adequate monitoring and treatment infrastructure means that chemical contaminants often go undetected, leading to severe public health issues [10]. The poor quality of drinking water in these countries exacerbates poverty, making it harder to achieve poverty alleviation goals and worsening overall deprivation [3]. As such, it is critical for these countries to focus their limited resources on managing the most harmful chemicals.

The World Health Organization (WHO) provides guidance on prioritizing certain chemicals, such as arsenic, nitrate, fluoride, and selenium, based on experience and known risks [11]. However, there has been a lack of research-based investigations to identify which specific chemical contaminants low-income countries should prioritize for risk management. This paper aims to fill this gap by identifying the most hazardous chemical contaminants globally to help guide decision-making in these countries.

Chemical contamination in drinking water has been a growing concern since the Industrial Revolution due to industrial chemical and fossil fuel emissions [12]. This issue has intensified over the past 50 years with rapid industrialisation and population growth [13]. Despite over 60,000 chemicals used industrially [14], only 200 chemicals have established guidelines from the WHO [11], and many unregulated chemicals continue to appear in water supplies [15]. In low-income countries, the extent of contamination is poorly understood due to limited monitoring [16], and hazardous chemicals like arsenic continue to be present in high concentrations in many regions [17].

Water safety planning (WSP) is considered the most effective method for managing drinking water risks [18]. WSP involves collecting data on health impacts, identifying causes of contamination, and prioritizing chemicals based on their risks [19]. Given [20] the regional variations in water quality and contamination sources, management strategies need to be adapted accordingly [6]. While alternative water sources and advanced treatment methods like Advanced Oxidation Processes (AOPs) are effective, they may be impractical for low-income countries due to high costs and technical demands [21,22,23,24].

Developing countries face numerous challenges in managing chemical contamination due to inadequate infrastructure, rapid industrialization, and limited information on emerging contaminants [20]. Waste mismanagement exacerbates environmental contamination [12], and studies from countries such as Malawi and Bangladesh have highlighted serious concerns regarding water quality and industrial pollution [25,26]. Additionally, many countries lack comprehensive regulations for inorganic chemicals compared to WHO guidelines, further complicating contamination management [27]. Limited funding for water quality surveillance and equipment is also a major issue, as seen in Brazil and Ecuador [28].

The overextraction of water resources, in which water is extracted faster than it can be replenished, can lead to water stress. This is particularly problematic in regions like West Asia and North Africa, where water stress levels have reached 81.5% [21]. Communities facing water stress may have to rely on contaminated sources due to a lack of alternatives. Furthermore, approximately 90% of the global population resides in countries that share transboundary water sources [29,30]. This interdependence can lead to conflicts if one country’s actions adversely affect shared water resources, as demonstrated by the tension over the Grand Ethiopian Renaissance Dam [31]. Additionally, the intrusion of saltwater into depleted aquifers can increase the concentration of harmful substances such as arsenic and fluoride [30].

Recent studies have highlighted instances of non-compliance and irregularities in the chemical composition of drinking water. For instance, research conducted in Malawi [32] and Libya [33] revealed discrepancies in the chemical composition of bottled drinking water. In India, studies on the Upper Gaoxi River [34] and Jharkhand State [35] identified parameters such as turbidity, E. coli, total coliform, and major ion chemistry exceeding acceptable limits, indicating compliance failures. Moreover, studies in the United States have shown disparities in nitrate levels in drinking water, indicating challenges in adhering to health-based guidelines [36]. The complexity of monitoring and enforcing water quality standards is evident, with over 400,000 public water systems requiring regular sampling and testing for various contaminants [37]. Violations in the Safe Drinking Water Information System (SDWIS) include exceeding maximum contaminant levels for chemicals like arsenic and nitrate, highlighting ongoing compliance issues [38]. Furthermore, studies have emphasized the link between social vulnerability and drinking water quality violations, particularly in communities with naturally occurring contaminants [39]. Research in Ethiopia [40] and Nigeria [41] also identified quality issues in bottled water and community water sources, emphasizing the need for continuous monitoring and enforcement to protect public health. Overall, these studies emphasize the importance of stringent monitoring, enforcement, and management strategies to ensure compliance with drinking water quality parameters and safeguard public health.

The objectives of this paper are to (1) identify the most hazardous chemical pollutants globally, (2) evaluate the current quality of drinking water, and (3) investigate how human characteristics (e.g., ethnicity and sex) influence disease risk. This study utilizes a literature review based on PRISMA guidelines [42] to estimate contamination levels, focusing on chemicals like arsenic, nitrate, and fluoride. Special attention is given to vulnerable populations, especially infants and young children, who are at higher risk of adverse health effects due to their developmental stages [43]. This research aims to provide critical insights into prioritizing chemical contaminants and guiding effective risk management strategies in low-income countries.

2. Materials and Methods

To determine which chemicals pose the greatest risk to human health, a comprehensive literature review was conducted. This review was guided by the PRISMA method guidelines [42]. The process involved the following research questions:

“Which chemical contaminants in drinking water are most harmful to human health?”

“Which chemical contaminants are most likely to appear in drinking water?”

Table 1. Criteria for the literature review and their justification.

Criteria	Justification
The paper should report the concentration of chemical contaminants, the country of study, the water origin (e.g., tap or bottled water), and the cause of contamination.	Determining concentrations and the country of study helps identify regions lacking safe drinking water. Knowing the water origin and cause of contamination aids in formulating strategies to improve water quality.
The paper should inform the risks to human health.	Understanding health risks helps governments gauge the impact of chemical contaminants and develop effective risk management strategies.

details the criteria used for the review and their justifications.

The literature search was performed using the Scopus and PubMed databases. Keywords included “drinking water”; “tap water”; “potable water”; “bottled water”; “chemical contaminant”; “chemical contamination”; “concentration”; “human”; “health”; “water quality”. Each keyword was refined using Boolean operators “AND”/“OR” to include specific chemical substances.

A total of 89 articles were analyzed through the following phases:

-

Phase 1: Title analysis.

-

Phase 2: Abstract analysis.

-

Phase 3: Full-article analysis.

The following types of reports were excluded from the analysis:

-

Reports focusing on the contamination of private water sources, as these are typically not regulated by most countries.

-

Reports lacking data on the concentrations of specific chemicals.

-

Reports that did not clarify whether the water was used directly for drinking or was pre-treated.

3. Results and Discussion

3.1. Priority Chemical Contaminants

Figure 2 shows a bar chart illustrating the most frequently referenced chemical contaminants in drinking water. The chart indicates that arsenic, fluoride, and nitrate are the three most common contaminants. As a result, these three chemicals were prioritized.

This prioritization aligns with the WHO’s recommendation (see Section 1), which also suggests that selenium should be a priority. However, selenium did not appear prominently in the results, indicating a potential research gap. Therefore, it is strongly recommended that further studies be conducted to investigate the presence of selenium in drinking water.

3.2. Screening

Table 2. Summary of concentration of arsenic, nitrate, and fluoride by country and water type.

Chemical Contaminant	Country	Concentration			USA Standard [44]	WHO Standard [45]	Type of Water	Impact Factor	Number of Citations	Reference
		Minimum	Maximum	Mean
Arsenic (µg/L)	India	-	-	0.083	10	10	Untreated groundwater	8.3	8	[46]
		0.31	19.73	3.19			Untreated groundwater	4.6	8	[17]
	Spain	5.9	11.5	-			Bottled	6.1	14	[47]
		11.1	35.8	-			Drinking water treatment	3.057	4	[48]
	Poland	0.24	27.8	2.39			Bottled	4.9	0	[49]
		0.092	1.22	0.49			Bottled	4.4	8	[50]
	Pakistan	17	58.0	33			Untreated groundwater	4.4	17	[51]
		2.5	7.9	4.2			Untreated groundwater	4.6	7	[52]
	Thailand	0.01	8.87	1.31			Tap	11.1	27	[53]
	Romania	0.5	130.3	4.11			Bottled and tap	4.4	28	[54]
	Ecuador	0.05	38.18	-			Tap	-	3	[55]
Nitrate (mg/L)	Pakistan	0.1	70	8.88	10	50	Untreated groundwater	6.18	7	[56]
	India	2.84	81.5	37.55			Untreated groundwater	6.18	41	[57]
		11.23	844.0	134.58			Untreated groundwater	6.18	80	[58]
	Spain	0.71	2.9	-			Tap	4.4	8	[59]
	Morocco	1.0	270.1	63.7			Untreated groundwater	2.3	4	[60]
	Mali	11.05	23.4	-			Untreated groundwater	8.8	20	[61]
	Saudi Arabia	6.0	11.8	15.0-			Untreated groundwater	3.39	8	[62]
	South Africa	1.8	17.1	6.0			Untreated groundwater	6.18	127	[63]
Fluoride (mg/L)	India	0.079	4.0	1.5	4	1.5	Untreated groundwater	8.3	80	[58]
		-	-	1.29			Untreated groundwater	6.18	8	[46]
	Pakistan	0.06	7.9	1.06			Untreated groundwater	3.251	7	[56]
		0.5	30	-			Untreated groundwater	8.8	6	[64]
	Saudi Arabia	0.5	4.6	0.75			Untreated groundwater	6.18	8	[62]

shows the number of papers screened for arsenic, nitrate, and fluoride. Initially, there were 37 papers on arsenic, 29 on nitrate, and 23 on fluoride. Screening reduced the total number of papers by 73%, resulting in 11 papers on arsenic, 8 on nitrate, and 5 on fluoride (see Figure 3 for detailed results).

3.3. Contamination Levels

This section compares contamination levels with the standards set by the WHO [2] and the United States Environmental Protection Agency (USEPA) [44] to determine if the detected levels are safe (see Table 2 for detailed data). The WHO’s guidelines are used as they help countries develop their own regulations and serve as legal standards in countries lacking their own regulations [2]. The USEPA’s standards are included due to their global recognition and substantial funding and research backing. Both USEPA’s and WHO’s guidelines are commonly used benchmarks in literature. Figure 4, Figure 5 and Figure 6 visualize the maximum reported concentration for each country, providing clear comparisons against the safe threshold set by the USEPA and WHO.

3.3.1. Arsenic

Arsenic had the highest number of reports (11), followed by nitrate (8) and fluoride (5). Figure 4 shows that Romania, Ecuador, Pakistan, Poland, Spain, and India all have arsenic concentrations exceeding the safe threshold set by the USEPA and WHO (10 µg/L) [2,44]. Romania has the highest recorded arsenic level in this study (130.3 µg/L), indicating a severe health risk for residents in this region. However, the mean concentration (4.11 µg/L) is significantly lower than the highest recorded level, suggesting that while some areas have dangerous levels, most of the water in Romania is relatively safe. The same is true for Poland, where the maximum concentration (27.8 µg/L) is much higher than the mean (2.39 µg/L), indicating that although some regions face unsafe contamination, the majority of potable water is safe. This overall safety is likely due to the EU’s efforts to provide safe drinking water to 70 million people by 2030 [65].

Figure 4 shows that not all EU member states have mean concentrations within the safe threshold. In one region of Spain, no recordings of arsenic levels were recorded within the safe range. The recorded levels ranged from 11.1 µg/L to 35.8 µg/L, indicating a significant risk for the entire population in the study area. The paper suggested that anthropologic activities are the primary source of this contamination. Therefore, for the EU to achieve its goal of providing safe drinking water to its citizens by 2030, it must focus on managing these anthropologic sources.

Figure 4 shows that Pakistan has the highest mean concentration (33 µg/L), which is more than three times the safe limit set by the WHO and USEPA. This indicates that large portions of the population of Pakistan are exposed to dangerously high levels of arsenic. Pakistan consistently faces poor drinking water quality across all three priority chemicals due to inadequate regulation management [64]. Recent industrialization and urbanization have further exacerbated the country’s water quality issues [66].

3.3.2. Nitrate

Figure 5 shows that Pakistan, India, Morocco, Mali, and Saudi Arabia have nitrate concentrations exceeding the safe threshold set by the USEPA (10 mg/L) [44]. Among these countries, India, Morocco, and Pakistan also have concentrations above the threshold established by the WHO (50 mg/L) [2]. The graph indicates that India has the highest mean nitrate level (134.58 mg/L), more than twice the mean level of the next most contaminated country (Morocco). India’s mean nitrate concentration, reported in one study, is over 13 times higher than the safe limit set by the USEPA and twice the limit set by the WHO. This suggests that people in some regions of India face a critical risk of health issues due to nitrate contamination.

This figure also reveals that Morocco has extremely high nitrate levels, reaching up to 270.1 mg/L. This is likely due to the area’s reliance on agriculture and animal farming, which involves the use of large quantities of nitrogen-rich fertilisers, pesticides, and animal waste [67]. The lack of sewage treatment facilities further exacerbates the contamination.

In Spain and Pakistan, it was observed that chemical concentrations were higher in shallow aquifers (<50 m) compared to deep aquifers (>100 m) [48]. This trend is attributed to shallow aquifers being more directly exposed to contamination sources, a pattern confirmed by multiple studies in this research.

3.3.3. Fluoride

Figure 6 shows that India, Pakistan, and Saudi Arabia all have concentrations exceeding the safe threshold set by the WHO (1.5 mg/L) [2]. Among these countries, Pakistan and Saudi Arabia reported concentrations surpassing the USEPA’s threshold (4 mg/L) [44]. Notably, Pakistan had the highest fluoride contamination level at 30 mg/L, which is 20 times greater than the WHO standard and 7.5 times higher than the USEPA standard. This indicates that communities in Pakistan face a critical risk of developing health issues due to fluoride contamination. The fact that 80% of Pakistan’s population lacks access to safe drinking water highlights the severe water quality problems in the country [66].

Table 2 indicates that no mean fluoride values recorded in the study exceed the standard set by the USEPA and WHO. This suggests that, overall, most communities in this study are not at significant risk of health problems from fluoride contamination.

Table 3 shows the potential exposure to arsenic, nitrate, and fluoride for infants (<1 year) and children aged 1 to 10, based on the literature review. For simplicity, the estimated amount of each chemical element ingested, measured in µg for arsenic and mg for nitrate and fluoride (denoted as A), is calculated using the product of the concentration (C) of each chemical (in µg/L for arsenic and mg/L for nitrate and fluoride), the daily water consumption (WC) per kg of body weight (L/kg), and the average weight (W) in kg for children aged six months to 5 years for both boys and girls, according to WHO Z-scores [42]. This calculation follows Equation (1) and is adapted from Campos et al. [68], who calculated sanitation risks based on hazard, exposure, and vulnerability. In this context, we use C, WC, and W as proxy indicators for hazard, exposure, and vulnerability.

A = C × WC × W

(1)

It is important to note that this method is a simplified approach. It does not account for (1) the specific form of arsenic, nitrate, or fluoride consumed, (2) the bioavailability of these chemicals, or (3) pharmacokinetics. The bioaccumulation of chemicals in an organism is complex [69] and requires consideration of both bioavailability and the known elimination half-lives of each contaminant. Future research should address these factors to better understand the impact of arsenic, nitrate, and fluoride on human health, particularly in infants.

Regarding maximum exposure values, the WHO limits are 10 µg/L for arsenic, 50 mg/L for nitrate, and 1.5 mg/L. Only Thailand fell below the exposure limit for arsenic. Spain, a developed country, had exposure values for children under 6 months that were approximately 3.6 times the WHO limit for boys and just over 3 times for girls. Romania showed values around 13 times higher than the limit for both boys and girls under 6 months. For nitrate, Spain, Mali, Saudi Arabia, and South Africa were below the WHO limit, while India had values about 17 times higher than the limit for 5-year-old children. For fluoride, all three countries analyzed had accumulated amounts above WHO standards for all ages up to 5 years, with Pakistan having levels approximately 20 times above the WHO limit for all ages.

Table 3. Exposure to arsenic, nitrate, and fluoride potentially accumulated by infants (<1 year) and children aged 1 to 10, based on the literature review.

	Arsenic
Country	Daily water consumption (WC) per kg of body weight L/kg [70]
		Infants (<1 year) 0.044			Children (1 to 10 years) 0.036
	Average weight (W) in kg of children aged six months to 5 years for boys and girls [71]
		Boys			Girls
		Six months	Two Years	Five Years	Six Months	Two Years	Five years
		7.9 Exposure Level (µg) 3.47 *	12.2 Exposure Level (µg) 4.39 *	18.3 Exposure Level (µg) 6.58 *	7.3 Exposure Level (µg) 3.21 *	11.5 Exposure Level (µg) 4.14 *	18.2 Exposure Level (µg) 6.55 *
	* Value calculated considering the maximum exposure limit for arsenic according to WHO—(10 µg/L)
	Maximum arsenic concentration found per country (C) µg/L	Amount (A) (µg) of arsenic in the infant’s and child’s body based on water consumption by age, weight and sex. A = C × WC × W
India	19.73	6.85	8.66	12.99	6.33	8.17	12.92
Spain	35.80	12.44	15.72	25.58	9.97	14.82	23.45
Poland	27.80	9.66	12.20	18.31	8.92	11.50	18.21
Pakistan	58.00	20.16	25.47	38.21	18.63	24.01	38.00
Thailand	8.87	3.08	3.89	5.84	2.85	3.67	5.81
Romania	130.30	45.29	57.22	85.84	41.85	53.94	85.37
Ecuador	38.18	13.27	16.76	25.15	12.26	15.80	25.01
	Nitrate
Country	Daily water consumption (WC) per kg of body weight L/kg [70]
		Infants (<1 year) 0.044			Children (1 to 10 years) 0.036
	Average weight (W) in kg of children aged six months to 5 years for boys and girls [71]
		Boys			Girls
		Six months	Two Years	Five Years	Six Months	Two Years	Five Years
		7.9 Exposure Level (mg) 17.38 *	12.2 Exposure Level (mg) 21.96 *	18.3 Exposure Level (mg) 32.94 *	7.3 Exposure Level (mg) 16.06 *	11.5 Exposure Level (mg) 20.70 *	18.2 Exposure Level (mg) 32.76 *
	* Value calculated considering the maximum exposure limit for nitrate according to WHO—(50 mg/L)
	Maximum nitrate concentration found per country (C) mg/L	Amount (A) (mg) of nitrate in the infant’s and child’s body based on water consumption by age, weight and sex. A = C × WC × W
Pakistan	70	24.332	30.74	46.12	22.48	28.98	45.86
Indian	844	293.37	370.68	556.02	271.09	349.42	552.98
Spain	2.865	0.99	1.26	1.88	0.92	1.18	1.87
Morocco	270.1	93.88	118.62	177.94	86.75	111.82	176.96
Mali	23.4	8.13	10.28	15.41	7.52	9.68	15.33
Saud Arabia	11.75	4.08	5.16	7.74	3.77	4.86	7.69
South Africa	17.1	5.94	7.51	11.26	5.49	7.08	11.20
Fluoride
Country	Daily water consumption (WC) per kg of body weight L/kg [70]
		Infants (<1 year) 0.044			Children (1 to 10 years) 0.036
	Average weight (W) in kg of children aged six months to 5 years for boys and girls [71]
		Boys			Girls
		Six months	Two Years	Five Years	Six months	Two Years	Five Years
		7.9 Exposure Level (mg) 0.52 *	12.2 Exposure Level (mg) 0.65 *	18.3 Exposure Level (mg) 0.98 *	7.3 Exposure Level (mg) 0.48 *	11.5 Exposure Level (mg) 0.62 *	18.2 Exposure Level (mg) 0.98 *
	* Value calculated considering the maximum exposure limit for fluoride according to WHO—(1.5 mg/L)
	Maximum arsenic concentration found per country (C) mg/L	Amount (A) (µg) of arsenic in the infant’s and child’s body based on water consumption by age, weight and sex. A = C × WC × W
India	1.792	0.62	0.78	1.18	0.57	0.74	1.17
Pakistan	30	10.42	13.17	19.76	9.636	12.42	19.65
Saudi Arabia	4.6	1.59	2.02	3.03	1.47	1.90	3.01

Table 3. Exposure to arsenic, nitrate, and fluoride potentially accumulated by infants (<1 year) and children aged 1 to 10, based on the literature review.

	Arsenic
Country	Daily water consumption (WC) per kg of body weight L/kg [70]
		Infants (<1 year) 0.044			Children (1 to 10 years) 0.036
	Average weight (W) in kg of children aged six months to 5 years for boys and girls [71]
		Boys			Girls
		Six months	Two Years	Five Years	Six Months	Two Years	Five years
		7.9 Exposure Level (µg) 3.47 *	12.2 Exposure Level (µg) 4.39 *	18.3 Exposure Level (µg) 6.58 *	7.3 Exposure Level (µg) 3.21 *	11.5 Exposure Level (µg) 4.14 *	18.2 Exposure Level (µg) 6.55 *
	* Value calculated considering the maximum exposure limit for arsenic according to WHO—(10 µg/L)
	Maximum arsenic concentration found per country (C) µg/L	Amount (A) (µg) of arsenic in the infant’s and child’s body based on water consumption by age, weight and sex. A = C × WC × W
India	19.73	6.85	8.66	12.99	6.33	8.17	12.92
Spain	35.80	12.44	15.72	25.58	9.97	14.82	23.45
Poland	27.80	9.66	12.20	18.31	8.92	11.50	18.21
Pakistan	58.00	20.16	25.47	38.21	18.63	24.01	38.00
Thailand	8.87	3.08	3.89	5.84	2.85	3.67	5.81
Romania	130.30	45.29	57.22	85.84	41.85	53.94	85.37
Ecuador	38.18	13.27	16.76	25.15	12.26	15.80	25.01
	Nitrate
Country	Daily water consumption (WC) per kg of body weight L/kg [70]
		Infants (<1 year) 0.044			Children (1 to 10 years) 0.036
	Average weight (W) in kg of children aged six months to 5 years for boys and girls [71]
		Boys			Girls
		Six months	Two Years	Five Years	Six Months	Two Years	Five Years
		7.9 Exposure Level (mg) 17.38 *	12.2 Exposure Level (mg) 21.96 *	18.3 Exposure Level (mg) 32.94 *	7.3 Exposure Level (mg) 16.06 *	11.5 Exposure Level (mg) 20.70 *	18.2 Exposure Level (mg) 32.76 *
	* Value calculated considering the maximum exposure limit for nitrate according to WHO—(50 mg/L)
	Maximum nitrate concentration found per country (C) mg/L	Amount (A) (mg) of nitrate in the infant’s and child’s body based on water consumption by age, weight and sex. A = C × WC × W
Pakistan	70	24.332	30.74	46.12	22.48	28.98	45.86
Indian	844	293.37	370.68	556.02	271.09	349.42	552.98
Spain	2.865	0.99	1.26	1.88	0.92	1.18	1.87
Morocco	270.1	93.88	118.62	177.94	86.75	111.82	176.96
Mali	23.4	8.13	10.28	15.41	7.52	9.68	15.33
Saud Arabia	11.75	4.08	5.16	7.74	3.77	4.86	7.69
South Africa	17.1	5.94	7.51	11.26	5.49	7.08	11.20
Fluoride
Country	Daily water consumption (WC) per kg of body weight L/kg [70]
		Infants (<1 year) 0.044			Children (1 to 10 years) 0.036
	Average weight (W) in kg of children aged six months to 5 years for boys and girls [71]
		Boys			Girls
		Six months	Two Years	Five Years	Six months	Two Years	Five Years
		7.9 Exposure Level (mg) 0.52 *	12.2 Exposure Level (mg) 0.65 *	18.3 Exposure Level (mg) 0.98 *	7.3 Exposure Level (mg) 0.48 *	11.5 Exposure Level (mg) 0.62 *	18.2 Exposure Level (mg) 0.98 *
	* Value calculated considering the maximum exposure limit for fluoride according to WHO—(1.5 mg/L)
	Maximum arsenic concentration found per country (C) mg/L	Amount (A) (µg) of arsenic in the infant’s and child’s body based on water consumption by age, weight and sex. A = C × WC × W
India	1.792	0.62	0.78	1.18	0.57	0.74	1.17
Pakistan	30	10.42	13.17	19.76	9.636	12.42	19.65
Saudi Arabia	4.6	1.59	2.02	3.03	1.47	1.90	3.01

3.4. Daily and Annual Potential Exposure

Appendix A presents the maximum daily and yearly intake of each chemical based on different characteristics.

Table A1. Exposure to priority chemicals in a day and year based on characteristics.

Chemical	Characteristic *		Maximum Value
			Day	Year
Arsenic (µg)			Romania
	Age Group	Infants (<1 year)	39.35	14,362.97
		Children (1 to 10 years)	95.90	35,003.79
		Teenagers (11 to 19 years)	125.74	45,894.92
		Adults (20 to 64 years)	177.99	64,966.28
		Adults (≥65 years)	190.11	69,389.31
	Ethnicity	Black	242.23	88,413.11
		White	215.39	78,615.85
		Hispanic	236.76	86,415.61
		Other	236.23	86,225.37
	Sex	Men	234.80	85,702.22
		Women	216.82	79,139.01
Nitrate (mg)			India
	Age Group	Infants (<1 year)	254.89	93,034.12
		Children (1 to 10 years)	621.18	226,732.16
		Teenagers (11 to 19 years)	814.46	297,277.90
		Adults (20 to 64 years)	1152.90	420,809.96
		Adults (≥65 years)	1231.40	449,459.54
	Ethnicity	Black	1395.13	509,223.18
		White	1569.00	572,683.54
		Hispanic	1533.55	559,745.02
		Other	1530.17	558,512.78
	Sex	Men	1520.89	555,124.12
		Women	1404.42	512,611.84
Fluoride (mg)			Pakistan
	Age Group	Infants (<1 year)	8.91	3251.79
		Children (1 to 10 years)	21.71	7924.88
		Teenagers (11 to 19 years)	28.47	10,390.64
		Adults (20 to 64 years)	40.30	14,708.41
		Adults (≥65 years)	43.04	15,709.78
	Ethnicity	Black	48.76	17,798.68
		White	54.84	20,016.78
		Hispanic	53.60	19,564.55
		Other	53.48	19,521.48
	Sex	Men	53.16	19,403.04
		Women	49.09	17,917.12

* Drinking water consumption data on age groups, sex and ethnicity is from the USEPA environmental factors handbook: 2011 edition [70].

indicates that adults aged 20 to 64 consume the highest concentration of arsenic annually. One study from India reported that adults in this age group ingest 450 g of nitrate in drinking water each year [64]. Pakistan reports the highest fluoride concentration, with adults consuming 15.7 mg of fluoride annually [66].

Figure 7 illustrates that infants consume more water per kg of body weight compared to adults. Infants consume 54.5% more water per kilogram of body weight than adults, leading to greater bioaccumulation of chemicals in their bodies. This increased intake makes chemical contamination particularly toxic for infants. Additionally, bioaccumulation can occur before and shortly after birth, as toxins are transferred from the mother via the umbilical cord and breast milk [43]. These toxins are especially harmful during infancy due to ongoing brain [72], hormonal, and bone development [43].

Figure 8 presents daily drinking water consumption across different ethnicities. It shows that Black individuals consume 11% more water daily compared to White individuals. This higher consumption suggests that the Black population may be more exposed to chemical contamination, potentially placing them at greater risk. With 1.5 billion people living in Africa [53], and considering the relationship between economic conditions and water quality, poor water quality is likely widespread across the continent. More research is recommended in Africa to better understand the extent of water quality issues there.

Additionally, Figure 8 indicates that Hispanic individuals are also at significant risk, consuming 9% more water daily than White individuals. South America with a population of 442 million [73], includes 67% low- and middle-income countries [74], which likely face significant challenges related to water quality. This concern is underscored by the fact that only 21% of South Americans have access to safe drinking water [75]. Therefore, further research on the water quality in South America is recommended to better assess the chemical contamination risks to human health in the region.

Currently, there are no specific and reliable data on daily water consumption in Asia. However, 58% of contamination reports originate from Asian countries, which have a combined population of 4.8 billion [76]. More detailed data on water consumption in Asia would provide a more comprehensive understanding of health risks related to water quality in low-income countries. Consequently, additional research into daily water intake in Asia is recommended.

Figure 9 shows that men consume 9.6% more water than women. However, when adjusting for body weight, the difference in daily water intake between men and women is minimal (0.025 L/kg for men and 0.028 L/kg for women). This suggests that chemical exposure for both men and women is likely similar, and the health impact for both sexes may be comparable.

3.5. Regional Distribution of Contamination

Figure 10 shows that India and Pakistan are the most studied countries, with six and five reports, respectively. Both countries have the largest populations in this study, indicating they bear the highest burden of chemical contamination.

Figure 11 shows that both countries are exposed to all three priority chemicals (arsenic, fluoride, and nitrate), making them the only countries in this study with such widespread contamination (see Figure A1 for the frequency of each type of chemical in each country). This extensive exposure is due in part to poor enforcement of regulations, inadequate wastewater treatment facilities, improper disposal of animal waste, and a high number of industries that do not treat industrial effluent [77,78]. Additionally, India’s contamination levels are exacerbated by its large agricultural sector, which uses 17% of the world’s nitrogen-based fertilisers [79].

India, Pakistan, Thailand, Ecuador, Mali, Morocco, South Africa, and Iran are classified as low-income countries [74] and account for 67% of the regional studies. This suggests that low-income countries face higher levels of priority chemical contamination. This is often due to the lack the effective regulations and infrastructure needed to provide safe drinking water [43]. Poorly enforced regulations result in more frequent industrial waste discharge into rivers, contributing to chemical contamination. These contaminants can also leach into surrounding groundwater as chemicals in rivers permeate the soil [80]. Furthermore, as manufacturing has shifted from developed countries to low-income countries, industrial waste has increased in these regions [81]. Table 2 reflects this trend, showing higher concentrations of the chemicals in low-income countries compared to developed countries. Despite being legacy contaminants, arsenic, nitrate, and fluoride remain a significant concern in low-income countries.

Figure 12 illustrates the disparity in the number of reports for each chemical in high- and low-income countries. The data show that the proportion of reports for arsenic is similar across income levels, suggesting that research is conducted at comparable levels in both high- and low-income countries. Additionally, this figure also suggests that arsenic pollution in high-income countries is likely due to naturally occurring arsenic, as these countries generally have adequate wastewater treatment facilities. For example, most arsenic contamination found in groundwater in the USA results from natural occurrences of arsenic [82]

Nitrate shows the most significant difference in reports, indicating that the poor wastewater treatment facilities in low-income countries are a major source of nitrate contamination. This issue is widespread, as highlighted by the fact that Latin America and the Caribbean, Sub-Saharan Africa, and South Asia (regions predominantly composed of low-income countries) manage only 40%, 24%, and 37% of their sewage safely, respectively [83]. Additionally, the figure reveals a notable difference in the number of fluoride reports for fluoride between high- and low-income countries. This aligns with other research indicating that most geogenic fluoride contamination in groundwater occurs in Asian and African regions [84].

Figure 13 shows that most reports originate from untreated groundwater sources, with 88% coming from low-income countries. This indicates that groundwater is the primary source of chemical contamination in these countries. This trend is particularly evident for arsenic and nitrate, with 88% and 100% of reports, respectively, related to groundwater. Low-income countries often rely on groundwater as their main source of drinking water because it typically contains fewer pathogens [5]. This reliance has created a mistaken belief that groundwater is “safe to drink” [63]. Consequently, the public in India and Pakistan may be unaware of the risks associated with chemically contaminated drinking water [57]. Additionally, low-income countries frequently use untreated groundwater because advanced water treatment technologies are unaffordable. Since groundwater is susceptible to both natural and anthropogenic pollution [63], it is often contaminated with harmful chemicals, making it unsafe to drink. Furthermore, Figure 13 shows that fluoride reports come from bottled water. This is likely because some countries add small concentrations of fluoride to drinking water in treatment plants to promote dental health [85]. Some studies have shown that the concentration of fluoride in most bottled waters is less than 0.3 mg/L [45], which is lower than the WHO-recommended value of 1.5 mg/L [45].

The reviewed papers also indicated that high levels of arsenic and nitrate in rural areas were primarily due to their presence in fertilizers, pesticides, and herbicides [48,56]. In contrast, urban areas had higher concentrations of these priority chemicals due to a variety of pollution sources concentrated in small areas, such as industrial effluent, sewage, and landfills [56].

3.6. Research Trend

Figure 14 shows the total number of papers published each year after screening. The number of publications has increased since 2018, with the highest number published in 2023. This trend suggests that chemical contamination of drinking water is becoming an increasingly significant concern. This rise in publications may be attributed to the declining quality of water [86] and growing public awareness of its impacts. Increased media coverage, informal networks, and individuals’ heightened sensitivity to water quality issues likely contribute to this growing concern [87].

3.7. Causes of Contamination

Natural arsenic contamination arises from geothermal activity and the dissolution of arsenic-rich geological formations into groundwater, either through direct contact or weathering) [17,46]. In Ecuador, it was observed that arsenic concentrations decreased during months of high rainfall due to the dilution of water sources [44]. This trend is common in studies on arsenic, nitrate, and fluoride contamination.

Arsenic contamination can also result from a variety of anthropogenic activities, such as the leakage of human waste [88], unregulated industrial activity, and illegal mining (as seen in Ecuador) [78]. The analysis of sediments reveals a significant increase in arsenic levels over the past 20 years [78], indicating that human activities have exacerbated arsenic contamination. Unregulated industries and illegal mining operations often release untreated arsenic-rich effluent in rivers, significantly contributing to the contamination of water sources [44]. In countries like Pakistan and India, high levels of chemical contamination are linked to the overexploitation of natural resources. Additionally, arsenic contamination is prevalent in rural areas where farms use arsenic-containing fertilisers, pesticides, and herbicides. This arsenic leaches into the groundwater, contaminating it. Many countries also reported that the overextraction of surface water, such as for irrigation, reduces the water supply and increases the concentration of chemicals in remaining water sources [58].

Similar to arsenic, nitrate contamination occurs naturally in drinking water through the dissolution of minerals high in ammonium into groundwater and the weathering of minerals [78]. However, pollution from nitrates in fertilisers, herbicides, and pesticides is the primary cause of nitrate contamination in drinking water [89]. Nitrates easily travel through the soil and leach into groundwater [88]. Contamination can also result from faecal matter (both human and animal), with the leakage of sewage systems and septic tanks posing a significant threat [90]. For example, in Mali, a pit latrine located just 30 m from a groundwater source led to elevated nitrate concentrations [61]. Furthermore, contamination from animal waste, herbicides, pesticides, and fertilisers tends to increase during the rainy season due to surface runoff [89].

Natural fluoride contamination is caused by the dissolution of fluoride-containing minerals in rocks into groundwater. These rocks include amphibole, topaz, phosphorite, biotite, mica, fluorite, and apatite [78,91]. Volcanic ash, which is high in fluoride and highly soluble in water, is another natural source of fluoride contamination [89]. Most fluoride contamination in drinking water, however, is due to anthropogenic activities, including sewage, air pollution, and effluent from factories involved in the production of bricks, plastics, glass, and other materials [64,89].

3.8. Impact on Human Health

Effective risk management strategies must consider the health risks associated with chemical contaminants [92]. To assist governments in developing such strategies, this report examines the impacts of arsenic, nitrate, and fluoride on human health.

Ingesting water contaminated with arsenic is linked to several types of cancer, including lung, skin, bladder, liver, prostate, and kidney cancer [93]. Arsenic exposure can also cause non-carcinogenic effects such as hyperpigmentation, skin lesions, cardiovascular problems, kidney damage, and harm to the endocrine, reproductive, hepatic, nervous, immune, renal, and respiratory systems [17,49,53]. Additionally, excessive arsenic exposure can lead to headaches, nausea, vomiting, diarrhoea, anaemia, and abdominal pain [94]. Research has shown that naturally occurring arsenic poses less health risk compared to inorganic arsenic [50].

Consuming nitrate-contaminated water has been associated with several health issues, including colon cancer, digestive system cancer [95], neural tube defects, thyroid disorders, central nervous system tumours, intrauterine growth restriction, and blue baby syndrome [60,62].

Drinking water with high fluoride concentrations can lead to dental and skeletal fluorosis, with children being particularly vulnerable [62,64]. Fluoride exposure can also damage the respiratory, excretory, reproductive, nervous, and digestive systems [96].

While high concentrations of these priority chemicals are strongly linked to various diseases, even low-level exposure over extended periods can increase the risk of developing the aforementioned health issues [97].

4. Future Perspectives

Climate change can significantly impact water quality [98]. Decreased rainfall due to climate change [99] reduces the water content in aquifers and rivers, leading to higher concentrations of chemicals in the water.

In some regions such as the UK and the Netherlands, climate change has increased rainfall, resulting in more surface runoff and higher levels of chemical contaminants in water sources [100]. For instance, in the UK, frequent flooding has overwhelmed wastewater treatment facilities, with 504 incidents in one year [100]. Consequently, more chemicals leach into water sources through surface runoff. Flooding can also displace water sources, altering the chemical composition of drinking water [99]. Furthermore, increased rainfall raises groundwater levels, which can mobilise a greater volume of chemical contaminants in the soil. The effects of climate change are likely to be exacerbated by population growth [101].

Despite a recent surge in research on drinking water chemicals, the impact of climate change on water quality is not yet fully understood [43]. Therefore, addressing climate change is an emerging priority that necessitates additional legislative changes.

5. Conclusions

This review identifies the most concerning chemical contaminants in drinking water for human health, with a primary focus on arsenic, nitrate, and fluoride. The literature review indicates that these contaminants are prevalent in drinking water sources globally, with particularly high concentrations in low-income countries. The primary sources of contamination include both natural processes and human activities, such as industrial effluents, agricultural runoff, and improper waste disposal.

Arsenic has been detected in dangerous concentrations in countries such as Romania, Pakistan, and India. For example, Romania recorded arsenic levels as high as 130.3 µg/L, significantly exceeding the WHO and USEPA safety threshold of 10 µg/L. Chronic exposure is linked to various cancers and other severe health issues.

High levels of nitrate contamination were found in India and Morocco, with concentrations reaching up to 844 mg/L in India and 270.1 mg/L in Morocco. These levels far exceed the WHO guideline of 50 mg/L and the USEPA standard of 10 mg/L. Nitrate contamination is associated with significant health risks, including methemoglobinemia and various cancers.

Excessive fluoride concentrations were noted in Pakistan and India, with maximum levels of 30 mg/L in Pakistan, vastly surpassing the WHO standard of 1.5 mg/L and the USEPA standard of 4 mg/L. Chronic exposure to high fluoride levels leads to dental and skeletal fluorosis.

All three contaminants pose serious health risks, particularly to vulnerable populations such as infants and children. For instance, children in regions with high arsenic levels may be exposed to up to 85.84 µg/day, while the maximum safe intake is 10 µg/L. Long-term exposure, even at low levels, can lead to severe health outcomes.

The review highlights significant regional disparities, with 67% of contamination reports originating from low-income countries. These regions bear the brunt of contamination due to inadequate infrastructure, lack of regulatory enforcement, and limited resources for water quality management.

Groundwater is the primary source of contamination in low-income countries, with 88% of reports related to groundwater sources. Although often perceived as safe, groundwater frequently contains high levels of harmful chemicals, as shown by 100% of nitrate reports and 88% of arsenic reports related to groundwater contamination.

The review highlights the need for improved monitoring, stricter regulation, and effective management strategies to mitigate the health risks posed by chemical contaminants in drinking water. Governments and international bodies must prioritize resources and interventions to address this critical public health issue, particularly in low-income regions. Further research is recommended to close knowledge gaps, particularly concerning emerging contaminants and the long-term effects of chronic exposure.