Health Impact Assessment from Rice Straw Production in Cambodia

Abstract

Rice is a staple crop best known for its crucial role in feeding humans. In Cambodia, the human population depends on rice as its staple food. Increased rice production results in an increase in straw residue. The extensive residue caused by straw burning significantly contributes to high concentrations of air pollution, which are associated with critical health issues. As such, our objective in this study was to assess the human health impact of rice straw production in Cambodia. In this assessment, we focused on primary and secondary fine particulate matter formation (PM_2.5, NH₃, NO_x, and SO₂) from fertilizer, combustion of fuel in rice production processing, and burning of straw. We assessed endpoint impact by the following methods: (1) We estimated the proportion of rice straw from the annual rice production of the Ministry of Agriculture, Forestry, and Fisheries (MAFF) of Cambodia subjected to open burning in all 25 Cambodian provinces. (2) We calculated air pollutants emissions in terms of kilograms of pollutants per ton of rice produced by following the air pollutant emission inventory (EI) in the EMEP/EEA Guidebook for 2019, using data for fertilizer and fuel combustion that was applied in the field, based on a face-to-face survey of farmers. (3) We analyzed health impact in terms of DALYs using the characterization factor (CF) adapted from the ReCiPe 2016 v1.1 method. The results showed that the total health impact of rice straw production was 13,093.50 DALYs. Our analysis showed that open burning alone contributed 98.55% of the total health impact considered in this study, while fertilizers and combustion contributed only 0.12% and 1.33%, respectively. As the major of the total health impact arose from open burning, a policy of zero open burnings should be an effective way to reduce health impacts. These findings provide information for policymakers on how to alleviate air quality issues caused by the practice of open straw burning by adopting alternative techniques of rice straw management.

1. Introduction

The Cambodian economy is heavily dependent on agriculture, and recent agricultural breakthroughs have helped many people escape poverty [1]. Cambodia is one of the ten countries of the Association of Southeast Asian Nations (ASEAN), having joined the organization on 30 April 1999. The total population of Cambodia was estimated at 15.55 million in 2019, with average annual growth of 1.4% [2]. Rice is the main food in Cambodia, with an estimated domestic consumption of 4.2 million tons in 2021 [3]. According to the annual report of the Ministry of Agriculture, Forestry, and Fisheries (Cambodia), the total rice production in the country in 2021 was estimated as more than 12 million tons from a total farmed area of 3,497,682 hectares [4]. It is Cambodia’s most important agricultural export product, the primary source of crop value-added products, and the main driver of agricultural growth, with rice cultivation covering more than 80% of all farmed area in the country [5].

Haze is an airborne mixture containing enough smoke, dust, moisture, and vapor to reduce visibility and is characterized by a high concentration of particulate matter mainly made of fine particulate matter less than 10 mm, which is produced by large-scale forest and land fires [6]. Over the past two decades, there have been experiences of transboundary haze pollution caused by forest and land fires in the ASEAN countries [7]. Due to the air pollution from forest fires and land fires, ASEAN members including Cambodia have agreed and created the roadmap for ASEAN cooperation towards Transboundary haze pollution control (Haze-Free ASEAN by 2020) by focusing on indicators PM _2.5 and PM₁₀ [7]. Cambodia, a country in Indochina, is one of the world’s most active fire hotspots. This is a matter of serious concern due to the high population density being close to high-frequency fire zones [8]. The open burning of agricultural residue in Southeast Asia (SEA) accounts for 43% of the global total of open biomass burning (BB), which substantially contributes to air pollution [9,10]. Among the BB categories, agricultural residue contributes 23% of the worldwide biomass burning total [11]. Biomass burning is a source of global air pollution, which commonly contains primary particulate matter (PM_2.5 and PM₁₀) and secondary formations of NH₃, NO_x, and SO₂ [12].

In Cambodia and other ASEAN countries, during the dry season, the open burning of rice straw after harvest is common–a practice that may greatly increase ambient air pollution [13]. Diverse farming practices such as the open burning of residue biomass, combustion of diesel or gasoline, and the use of fertilizers, extending all the way from large, sensitive, and specialist commercial operations to subsistence-level cultivation, all result in PM_2.5 formation. The rice cultivation activities that generate pollutants such as sulfur oxide (SO_x), nitrogen oxide (NO_x), and ammonia (NH₃) play a key role in PM_2.5 formation. PM_2.5 is an aerodynamic particle with a diameter equal to or less than 2.5 μm, which impacts air quality, causes air pollution, and impacts human health [14]. Furthermore, Southeast Asia’s air pollution is strongly affected by biomass burning at all scales, from regional to local [15]. Ambient fine particulate matter (PM_2.5) is a key risk factor for health impacts and is likely to be harmful to humans, with extensive and well-documented concerns about disease reported in the literature [16]. Globally, 4.1% of deaths are attributed to indoor air pollution [17]. A WHO report on the impact of air pollution stated that the premature deaths of 2.2 million people every year are linked to air pollution in the home and outdoors. Among these causes of death are pneumonia (8%), ischemic heart disease (29%), stroke (27%), chronic obstructive pulmonary disease (22%), and lung cancer (14%) [18].

In Cambodia, rice straw is widely available and low in value. Without proper treatment, rice straw is left to burn in the field. After harvest, roughly 10 million tons of rice straw are produced in Cambodia, of which 3 million tons are burned [19]. Therefore, this practice results in higher greenhouse gas emissions, contributes to air toxicity, and is harmful to human health. Several alternative techniques for rice straw use have been proposed in Cambodia, such as its reuse as feed for mushroom cultivation, vegetable production, and the feeding of livestock. To improve facilities for rice straw waste management, Cambodia aims to (1) design and develop a prototype machine for collection purposes; (2) improve technology for mushroom and ruminant farming; (3) improve bioenergy technology; (4) conduct sustainability research and educate rice straw farmers on sustainable rice production; (5) produce a rice straw production and waste database, and a GIS map; and (6) introduce mechanized collection methods and supply chain models [19]. There was a study of pollutants emission from agricultural residue open burning in Southeast Asia, including Cambodia, where it was found that 117 Tg/year of rice straw was burned and rice straw made up 85–98% of the total CROB emissions, which was a significant contribution [20]. In recent years, there was a study of Health Impacts and Cost Assessment of Fine Particulate Matter Formation from Rice Straw Utilization in Thailand where straw open burning was the highest contributor to health impacts [21].

We had three main objectives in assessing the environmental impact of particulate matter (PM_2.5), of both primary and secondary formation, from rice straw production in Cambodia: (1) to quantify the amount of rice straw subjected to open burning in Cambodia; (2) to estimate the emission inventory (EI) from rice production processes and open rice straw burning in the 25 provinces of Cambodia; and (3) to calculate endpoint impact categories for Cambodia’s rice production and straw burning processes and convert these to human health impact units in disability-adjusted life years (DALYs), with a focus on air pollutants SO₂, NO_x, NH₃, and PM_2.5.

2. Materials and Methods

The summary of the overview research framework which divided into three phases as shown in Figure 1. We conducted this study on fine particulate matter formation produced from rice straw in the 25 provinces of Cambodia. We assessed the endpoint impact category within the ISO 14040:2006 standardized framework for the life cycle assessment (LCA) method [22].

2.1. Study Area

We assessed the health impact of rice straw production in Cambodia, which is an ASEAN country located to the northeast of Laos, with Thailand to the west and northwest, Vietnam to the east and southeast, and the Gulf of Thailand to the southwest as shown in Figure 2.

In this study, we considered the impact of rice straw production on human health at the provincial level in Cambodia. We analyzed the rice straw generated by the rice production process in Cambodia in 2021. We investigated the formation of particulate matter, both primary and secondary, and measured levels of fine particulate matter (PM_2.5), ammonia (NH₃), nitrogen oxide (NO_x), and sulfur dioxide (SO₂), all of which are air pollutants. We considered the straw production process in terms of fertilizer usage, combustion in rice farm activities, and open burning of straw. We measured air pollution emissions present in kilograms per ton of rice paddy. The collected data from the survey conducted by rice farmers in Cambodia mainly focused on the varieties of short-term rice (3-month period).

2.2. Rice Straw Generation

We obtained the source data of total rice production in the provinces of Cambodia from the Ministry of Agriculture, Forestry, and Fisheries. We calculated the total mass of rice straw based on rice paddy production by following the expression described in the Atmospheric Brown Clouds: Emission Inventory Manual [23], as follows:

M = P × S × D × B × η

(1)

where M is the amount of biomass of a certain crop residue that is openly burned each year (mass/year); P is the crop production (in mass per year); S is the specific crop resistance to production ratio; D is the dry-matter-to-crop residue ratio (fraction, 0–1); B is the fraction of dry matter of crop residue that is subjected to open burning; η is the percentage of burn efficiency particular to crops (fraction oxidized during combustion). We obtained these definitions from the literature [23].

By following the methods in previous studies, we determined S as 1.75 [24], D as 0.85 [25], and η as 0.89 [26]. The value of B varies with the specific conditions of individual countries [9]. However, for the ASEAN countries, we determined a probable value for B of 0.51, given previous recommendations [9]. Since previous study of the impact of rice straw burning in Cambodia was conducted for the country as a whole, there is to our knowledge no existing study regarding the impact at the provincial level. Thus, the application of burning friction was attributed the same value for all provinces.

2.3. Fertilizer Usage and Combustion in Rice Production

We conducted face-to-face field surveys with rice farmers to collect data on fertilizer use, diesel and gasoline use, and all activities relating to rice straw production activities. We determined the sample size using the Cochran formula [27]:

$${\mathrm{n}}_0=\frac{{\mathrm{Z}}^2\times \mathrm{p}\times \mathrm{q}}{{\mathrm{e}}^2}$$

(2)

$$\mathrm{n}=\frac{{\mathrm{n}}_0}{\left[1+\left\{\frac{({\mathrm{n}}_0-1)}{\mathrm{N}}\right\}\right]}$$

(3)

where: n is the sample size; Z² is the area under the acceptance region in a normal distribution (1 − α); e is the preferred level of precision; p is the estimated proportion that is present in the population which has an attribute in question, it describes the percentage value associated with population; and q = 1 − p. (proportion of the population not present in the population). Where n is the sample size: N is the population size of adjustment. This adjustment can substantially reduce the necessary sample size for small populations and is also called the population correction

2.4. Emission Inventory

Rice straw refers to agricultural waste. We calculated emission inventories for SO₂, NO_x, NH_3, and PM_2.5 in accordance with the European Environmental Agency (EMEP/EEA) guidelines; this guide book provided the specific pollutants emission from specific sectors [28], so that:

E = AD × EF

(4)

where E is the emission inventory; AD is the activities data; and EF is the emission factors determined using the EMEP/EEA method.

Table 1. Emission factors of pollutants from straw residue burning and particulate matter formation potential.

Emitted Substance	Emission Factor of Pollutants g/kg of Dry Straw	Particulate Matter Formation Potential (PMFP) kg Primary PM2.5 eq/kg
PM2.5	8.3	0.54
NH3	4.1	0.04
NOx	1.12	0.05
SO2	0.51	0.28

Emission factor of pollutants adopted from m N.T. Kim Oanh, et al. (2018) [20,29,30], and Particulate matter formation potential (PMFP) adopted from ReCiPe 2016 v1.1 [28].

is the pollutants emission factor of rice straw residue open burning and its potential to particulate matter PM2.5 formation that have applied for data calculation The effect due to exposure to PM_2.5 calculated by applying endpoint characterization (CF) was obtained from a global factor that was not developed specifically for Cambodia, and thus this application would be effective in calculating the result of endpoint impact on human health in this study case.

2.5. Human Health (DALYs)

As shown in Equation (4), we defined human health impact in DALYs after we calculated the pollutant emission results, as also shown in Equation (4). We used ReCiPe 2016 v1.1 characterization factors (CF) as part of a harmonized life cycle assessment of endpoint impact [31].

HI = E × CFs

(5)

where HI is the human health impact in DALYs; CFs are the characterization factors acquired using the ReCiPe 2016 method with endpoint characterization factors of PM_2.5, NH_3, NO_x, and SO₂ as 340, 23, 29, 160 (yr∙kton⁻¹), respectively.

Definition 1.

The loss of one DALY is equivalent to the loss of one year of full health. For any given disease or condition, DALYs are calculated by summing the years of life lost to premature death and the years spent with a disability as a result of the high prevalence in the population of the disease or condition of concern [32].

Table 2. Sources of input data for analysis.

Data Type	Data Used	Data Source
Primary Data	Whole process of Short-term rice production	Interview and Questionnaires (QN)
	Fertilizer usage for rice production	Interview and QN
	Diesel and Gasoline use for rice production	Interview and QN
	Farmers practice of rice straw Management	Interview and QN
Secondary Data	Pollutants Emission Inventory	EMEP/EEA-guidebook-2019
	Cambodia rice production 2021	Ministry of Agriculture, Forestry, and Fisheries

presented about summary of input data and its sources for data analysis in the study that were divided into two different data types (primary and secondary data). After the final analysis result by using ArcGIS tool Version 4.1, the distribution map of health impact map of each province has been produced.

3. Results and Discussion

3.1. Farmers Survey

We conducted a field survey in three provinces in Cambodia: farmers in Prey Veng, Svay Rieng, and Kampong Thom, which are the provinces that produce a large amount of rice production and where most of the population depends on rice farming. From the 90% confidence interval of the total population 2,259,242, the calculated result was that 67 respondents should be selected, and thus 70 farmers were interviewed. As a result, we found differences in the amounts of fertilizer and energy used, and how they managed rice straw after harvesting, all of which varied between farming areas. In practice, fertilizers applied to the field were inconsistent with one another. We found that the fertilizers most applied were urea (37%), NPK compound with 20% nitrogen (33%), and NPK compound with 27% nitrogen (11%). We determined the average amount of energy used for land operation, water pumping, and harvesting to be 165.05 L/ha of diesel and 8.64 L/ha of gasoline. For the purposes of data analysis, we note that fertilizer, diesel, and gasoline emission values (kg/ton of rice paddy) were not the same in the three selected provinces at the time of our study. Therefore, we determined a pollution emission factor as an average from the three selected provinces and applied this figure to the other 22 provinces in Cambodia, as shown in

Table 3. Pollutants emitted in rice production per data from the survey.

Pollutants Emission Factor (kg/ton Rice)
Pollutants	Prey Veng	Svay Rieng	Kampong Thom	Average	Emission Activities
PM2.5	0.021	0.021	0.021	0.02	Combustion
NH3	1.25	0.62	0.65	0.84	Fertilizer
NOx	0.96	0.54	0.57	0.69	Fertilizer & Combustion
SO2	0.22	0.22	0.22	0.22	Combustion

.

3.2. Pollutants Emission Inventory

We determined the total emissions of PM_2.5, both primary and secondary, in the 25 Cambodian provinces for three different categories: fertilizer usage, fuel combustion, and straw residue burning. The primary PM_2.5 was produced by the open burning of straw waste and fuel combustion during rice paddy production, and secondary formations were the result of fertilizer usage and open burning, which produced NH₃, NO_x, and SO₂ pollutants. We found that the annual amount of rice straw produced and subjected to burning in the field was approximately 8.24 million tons, while the amount of dry straw subjected to burning in the field was 675.1762 kg/ton of rice paddy. In addition, the average levels of air pollutants PM_2.5, NH₃, NO_x, and SO₂ emitted from fertilizer and fuel combustion were 0.02, 0.84, 0.69, and 0.22 kg/ton of rice paddy, respectively.

These results are shown in Table 3, which is a graph of the estimated pollutant levels emitted by various sources: rice straw residues (RSOB), fertilizer, and fuel combustion, presented as tons. Provinces that produced more rice production produced higher quantities of pollutants as well. The total emissions of air pollutants in the whole process of rice straw production for PM_2,5, NH₃, NO_x, and SO₂ were 37,414.53, 1713.23, 809.89, and 1935.43 tons, respectively. The emission factor of pollutants for particulate matter formation potential (PMFP) with respect to PM_2.5 from RSOB in Cambodia is shown in

Table A1. Particulate Matter Formation Potential (PMFP) of Rice Straw Open Burning in Cambodia.

Pollutants Emission Factor (kg/ton of Rice Paddy)
PM2.5	NH3	NOx	SO2
3.05	0.107	0.034	0.097

, and total pollutants emission is shown in

Table A2. Provincial level Emission Inventory and Health Impact from rice straw production in Cambodia.

Pollutants Emission Inventory (ton/year)					Health Impact (DALYs)
Provinces	PM2.5	NH3	NOx	SO2	PM2.5	NH3	NOx	SO2	Total HI
Banteay Beachey	5652.95	2819.39	792.77	408.76	1049.07	3.22	1.91	25.54	1079.74
Battam Bang	8503.84	4241.27	1192.58	614.90	1578.14	4.85	2.88	38.42	1624.28
Kampong Cham	3054.00	1523.17	428.29	220.83	566.76	1.74	1.03	13.80	583.33
Kampong Chhnang	3894.89	1942.57	546.22	281.64	722.81	2.22	1.32	17.60	743.95
kampong Speu	2274.29	1134.29	318.95	164.45	422.06	1.30	0.77	10.27	434.40
kampong Thom	5480.11	2726.05	763.21	396.26	1016.99	2.96	1.70	24.76	1046.41
Kampot	3015.16	1503.80	422.85	218.02	559.55	1.72	1.02	13.62	575.91
Kandal	2146.35	1070.49	301.00	155.20	398.32	1.22	0.73	9.70	409.96
Koh Kong	153.26	76.44	21.49	11.08	28.44	0.09	0.05	0.69	29.27
Kratié	981.17	489.36	137.60	70.95	182.09	0.56	0.33	4.43	187.41
Mondulkiri	487.87	243.32	68.42	35.28	90.54	0.28	0.17	2.20	93.19
Phnom Penh	101.74	50.74	14.27	7.36	18.88	0.06	0.03	0.46	19.43
Preah Vihear	1627.99	811.95	228.31	117.72	302.12	0.93	0.55	7.35	310.95
Prey Veng	8977.56	4503.05	1278.52	649.16	1666.05	5.71	3.60	40.56	1715.92
Pursat	3943.70	1966.91	553.06	285.17	731.87	2.25	1.33	17.82	753.27
Ratanakiri	417.81	208.38	58.59	30.21	77.54	0.24	0.14	1.89	79.80
Siem Reap	3222.18	1607.06	451.88	232.99	597.97	1.84	1.09	14.56	615.45
Preah Sihanuk ville	207.19	103.33	29.06	14.98	38.45	0.12	0.07	0.94	39.57
Steung Treaeng	420.60	209.77	58.98	30.41	78.05	0.24	0.14	1.90	80.34
Svay Rieng	3350.64	1665.97	465.87	242.28	621.81	1.79	1.02	15.14	639.75
Takeo	7131.86	3557.00	1000.17	515.70	1323.53	4.07	2.41	32.22	1362.23
Oudor Meanchey	1523.81	760.00	213.70	110.19	282.79	0.87	0.52	6.88	291.06
Kep	64.26	32.05	9.01	4.65	11.92	0.04	0.02	0.29	12.27
Pailin	139.05	69.35	19.50	10.05	25.81	0.08	0.05	0.63	26.56
Tbong Khmum	1774.96	885.26	248.92	128.35	329.40	1.01	0.60	8.02	339.03
Total	68,547.23	34,200.98	9623.23	4956.59	12,720.94	39.40	23.49	309.67	13,093.50

. and total pollutants emission is shown in Table A2. Prey Veng was the top province where produced pollutants from rice production and followed by Battam Bang and Takeo province that have shown in Figure 3.

The direct emission of PM_2.5 was caused by fuel combustion and the open burning of straw, accounting for 99.63% of the total generated from straw burning; the remaining 0.37% was generated by fuel combustion. We discovered that burning straw and applying fertilizer to the field caused the secondary PM_2.5 formation of NH₃. In this study, we also found another particulate matter formation (NO_x), of which 24.10% was generated by fertilizer, 24.35% by fuel combustion, and 51.56% by straw burning. Both burning of straw and fuel combustion produce SO₂, which is the other important particulate matter pollutant. A total of 52.08% of SO₂ emissions was attributed to burning straw and 38.92% to combustion. However, the pollutants emission factors from the straw open burning were adopted from the previous study which is not specific to Cambodia; due to this, the emission inventory analysis for Cambodia is not very accurate at the province level.

3.3. Health Impact (DALYs)

In this study, the total health impact of rice straw production in Cambodia was 13,093.50 DALYs. Rice production is the main causal factor in terms of provincial impact. Considering the impact on health, open burning contributed 98.50% of the total figure, which is consistent with the results of other studies in Thailand, where open burning of rice straw contributed 95% [21] of the impact overall. We found that fertilizer and diesel combustion did not have much influence on human health. The health impact from RSOB at the provincial level in Cambodia is shown in detail in Table A2.

Figure 4 illustrates how measurements of health impact varied in line with the total levels of rice produced in individual provinces. Only the process of open burning resulted in a serious health impact (12,904.69 DALYs), according to our results. In contrast, in terms of health impact, fertilizer and combustion contributed only 15.07 DALYs (0.12% of the total impact) and 173.74 DALYs (1.33% of the total), respectively. In addition, we applied a value for fraction of dry matter subjected to burning (Bk) to all 25 provinces at the same level (0.51), although the alternative straw management techniques actually practiced do not precisely match this definition. Thus, the value of B_k impacted the result of both pollutant emission and health impact. The characterization factor (CF) that we used to calculate health impact is a global factor that is not specific to Cambodia, and so would have had a notable influence on our health impact calculations. However, secondary PM_2.5 formation from fertilizer and combustion in the emission inventories of NH₃, NO_x, and SO₂ (presented in ton/year) contributed 23.89–38.02% of the total emission, but in line with the calculated HIs for each impact, we state that the open burning of straw contributed up to 98% of the total health impact.

Open burning was the main contributor to air pollution. In this study, we found that 98.50% of the total health impact was from the open burning of straw residue, while emissions from fertilizer and combustion did not produce any negative impacts on health. Some policies have been proposed and implemented to reduce these emissions. Improvements in straw usage rates and a decrease in open-field burning are just two of the suggestions for supporting a circular bio-economy that uses agricultural straw as a resource [33]. Another solution to avoid the open burning of straw is to recycle rice straw in soil as a potential step toward zero burning in the future. Recycling straw can increase the organic matter content in the soil by incorporation into the land [34]. In Cambodia, educating people not to open-burn straw may be the best solution to reducing air pollution [35].

4. Conclusions

We conducted a study of the endpoint health impact of fine particulate matter formed from rice straw production at the provincial level in Cambodia. We estimated the annual average quantity of dry straw production subjected to burning to be 8.24 million tons of rice production in 2021. We calculated the health impact (DALYs) using the global characteristic factor (CF) as 13,093.50 DALYs. The PM_2.5 formation contributed 12,904.69 DALYs (98.50%) of the total health impact, while fertilizer contributed 15.07 DALYs and fuel combustion contributed 173.74 DALYs. We conclude that fertilizer usage and combustion activities have no negative impact on human health. Therefore, open burning poses the main threat to human health. By reviewing alternative straw residue management techniques and considering zero-burning policies, future studies might propose scenarios that incorporate various residue management approaches and assign various amounts of crop residue to each methodology under consideration in the pursuit of recommendations for optimal scenarios of efficient rice straw residue management. In conclusion, the unspecified fraction of dry matter of crop residue that is subjected to open burning (B_k) at the individual provincial level in Cambodia should be developed in future studies. The findings in this study provide information for policymakers on how to alleviate air quality issues caused by the practice of open straw burning by adopting alternative techniques of rice straw management.