Tropospheric Ozone: A Critical Review of the Literature on Emissions, Exposure, and Health Effects

Abstract

Tropospheric ozone is a significant air pollutant with severe adverse effects on human health. The complex dynamics of ozone formation, distribution, and health impacts underscore the need for a comprehensive understanding of this pollutant. Despite well-documented health risks, including an estimated 423,100 deaths annually due to ozone exposure, millions of people in major countries continue to be exposed to unhealthy levels. Notably, the epidemiological evidence linking long-term ozone exposure to health outcomes is limited compared to short-term exposure studies, leaving some findings incomplete. Regulatory standards vary globally, with the implementation of the World Health Organization recommendation for an 8-h average limit of 50 ppb to protect public health remaining heterogeneous, leading to significant disparities in adoption across countries, and often significantly higher. Emissions from diesel and gasoline vehicles are major sources of VOCs and NOx in urban areas, and their reduction is a key strategy. Additionally, climate change may exacerbate ozone pollution through increased natural precursor emissions, leading to higher ground-level ozone in polluted regions, like the eastern US, southern Europe, and parts of Asia. Addressing tropospheric ozone effectively requires an integrated approach that considers both natural and anthropogenic sources to reduce concentrations and mitigate health impacts.

1. The Ozone Paradox: “Good” in the Stratosphere, “Bad” in the Troposphere

Ozone is a molecule formed by three oxygen atoms (O₃). It exists naturally in tiny quantities high up in the atmosphere, known as the stratosphere. Here, ozone acts as a guardian, filtering out the harmful ultraviolet rays from the sun, protecting life on Earth. However, closer to the ground, in the troposphere, ozone forms through chemical reactions between pollutants released by vehicles, fuel vapors, and various industrial processes. Unfortunately, at ground level, high concentrations of ozone become poisonous to both humans and plants.

1.1. Stratospheric “Good” Ozone

Ninety percent of our planet’s ozone resides in the stratosphere, a region stretching roughly 10 to 50 km above us. This natural ozone layer is a delicate balance between sunlight creating ozone and chemical reactions breaking it down. Sunlight acts like a knife, splitting regular oxygen molecules (O₂) into single oxygen atoms. These lone atoms can either recombine back into O₂ or partner with an existing O₂ molecule to form ozone (O₃). Unfortunately, ozone can also be destroyed by reactions with molecules containing nitrogen, hydrogen, chlorine, or bromine. While some of these ozone-destroying molecules occur naturally, human activities have introduced others.

High above us, ozone acts as a vital shield, absorbing the brunt of the sun’s harmful ultraviolet (UV) radiation. Without this protective layer, the Earth’s surface would be bombarded by intense UV rays, rendering it virtually sterile. Thankfully, ozone acts as a powerful filter, completely blocking out the most energetic UV-C radiation and absorbing most of the damaging UV-B rays. While it allows some UV-A radiation through, it still filters out a significant portion.

Humanity’s growing use of chlorofluorocarbons (CFCs) has disrupted the delicate balance in the stratosphere. These synthetic gases have accelerated the rate of ozone destruction, causing a concerning decline in its overall levels. This thinning of the ozone layer allows more harmful ultraviolet (UV) radiation to reach Earth’s surface.

When scientists refer to the “ozone hole”, they are specifically addressing this worrying depletion of the “good” ozone layer residing high up in the stratosphere.

The consequences of excessive UV-B and UV-A exposure are severe. These rays can cause painful sunburn and lead to serious health problems, like skin cancer and eye damage. By acting as a critical barrier, ozone safeguards life on Earth from the sun’s relentless assault [1].

1.2. Tropospheric “Bad” Ozone

While stratospheric ozone acts as a protective shield for life on Earth, ground-level ozone poses a significant threat to both plants and animals, including humans [2,3]. This “bad” ozone forms near the Earth’s surface in a toxic tango between nitrogen oxide gases (released from vehicles and industries) and volatile organic compounds (VOCs)—chemicals that readily evaporate into the air, like paint thinners.

Naturally, the troposphere (the lower atmosphere) contains minimal ozone, around 10 parts per billion (a mere 0.000001%). However, the US Environmental Protection Agency (EPA) warns that ozone concentrations exceeding 70 parts per billion for over 8 h pose a health risk [4]. These unhealthy levels often occur in or around cities during periods of warm, stagnant air. This “bad” ozone can irritate throats and lungs, worsening conditions like asthma and emphysema. A recent study found that over 4 out of 10 cities globally (more than 40%) faced unhealthy levels of ozone (O₃) ranging from 40 to 60 micrograms per cubic meter (µg/m³). These cities were mainly concentrated in China and India. The research also revealed significant seasonal variations in ozone levels worldwide. Summer had the highest average concentration (45.6 µg/m³), followed by spring (47.3 µg/m³), autumn (38.0 µg/m³), and winter (33.6 µg/m³). Looking at exposure, the analysis showed that roughly 12.2% of the population in 261 cities lived in areas with high ozone concentrations (80–160 µg/m³). This translates to approximately 36.32 million people in major countries being exposed to unhealthy ozone levels [5]. Regulatory standards and health guidelines vary across different global areas, with the WHO and India adopting stricter values to protect public health, which equate to 50 ppb over an 8-h average.

Table 1. Values for ozone in parts per billion (ppb) for various regions and averaging times.

Country or Region	Ozone Level (ppb)	Averaging Time
WHO	50	8 h
European Union	60	8 h
United State of Amrica	70	8 h
Unite Kingdom	50	8 h
Canada	62	8 h
China	100	1 h
	80	8 h
India	90	1 h
	50	8 h

shows regulatory values for ozone in parts per billion (ppb) for various regions and averaging times [6,7,8].

2. Emissions of Tropospheric Ozone

Tropospheric ozone, a powerful oxidant and major component of photochemical smog, is not emitted directly into the atmosphere. Rather, it is formed through complex chemical reactions involving ozone precursors, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs), in the presence of sunlight. The sources of these precursors can be either natural or anthropogenic, both contributing significantly to the formation of ambient ozone.

2.1. Natural Sources

Natural emissions of ozone precursors include biogenic VOCs, emitted mainly by vegetation, and NOx produced by natural events, such as lightning and soil microbial activities. Forests, in particular, release a variety of VOCs, such as isoprene and monoterpenes, which are crucial for ozone formation. For example, tropical forests are known to emit large quantities of isoprene, which reacts rapidly with NOx in the presence of sunlight, significantly increasing ozone concentrations. In addition, lightning produces NOx through the thermal dissociation of nitrogen molecules, which then combine with oxygen to form nitrogen oxides, an important precursor to ozone. Oceanic and soil processes also contribute, albeit to a lesser extent, to natural VOC and NOx emissions. A recent review indicated that future research on the factors affecting the formation of NOx and VOCs in the soil environment is essential for enhancing our understanding of the role of the soil environment in the formation of ground-level ozone [9]. Climate change could exacerbate ozone pollution through enhanced natural precursor emissions, increased stratospheric-tropospheric exchange, stagnant weather, methane emission changes, and elevated carbon dioxide levels [10]. Future climate change is expected to significantly impact ozone levels, especially due to rising temperatures and increased moisture in the atmosphere. This could lead to higher ozone concentrations near the ground in polluted regions, like eastern US, southern Europe, and parts of Asia. However, the understanding of how climate change will affect ozone levels is incomplete, and existing models may not fully capture the complex interactions at play [11].

2.2. Anthropogenic Sources

Human activities are a significant source of NOx and VOCs, contributing significantly to tropospheric ozone formation, especially in urban and industrialized areas. Diesel and gasoline vehicles emissions are a major source of VOCs and NOx in cities, where heavy traffic releases large quantities of these precursors (5). Figure 1 shows the source contributions of the six identified factors for VOCs and NOx in a typical industrial city in China, which are industrial process, biogenic source, coal combustion, solvent utilization, diesel vehicle emission, and gasoline vehicle emission. In this study, diesel and gasoline vehicle emissions were the major contributors to VOCs (44.60%) and NOx (45.70%) [12]. Motor vehicles are a major source of NOx and VOCs in cities, where heavy traffic releases large quantities of these precursors [13]. Industry and energy production, through the combustion of fossil fuels, are another important source of NOx [14]. Industrial emissions also include VOCs from processes such as oil refining, chemical production, and other manufacturing activities [15]. In residential and commercial areas, heating and the use of solvents and other chemicals further contribute to VOC emissions [16].

2.3. Photochemical Reactions

The formation of ozone in the lower atmosphere is the result of a series of photochemical reactions involving precursors emitted from both natural and anthropogenic sources. These reactions begin with the photolysis of nitrogen dioxide (NO₂) under sunlight, which produces atomic oxygen (O). This atomic oxygen then reacts with molecular oxygen (O₂) to form ozone (O₃). VOCs play a crucial role in this process, reacting with free radicals to produce additional NO₂, which further fuels ozone production in a continuous cycle.

2.4. Spatial and Temporal Variations

Ozone concentrations vary significantly depending on time and location. During the summer months, when sunlight is more intense and temperatures are higher, photochemical reactions are faster, leading to higher ozone levels. Urban areas, with high concentrations of traffic and industrial activities, tend to have higher ozone levels than rural areas, although long-distance transport can carry ozone and its precursors to remote regions, contributing to high ozone levels even far from emission sources. While ozone levels have decreased in rural areas since the 1990s, they have conversely increased in urban areas, with a stronger trend toward these contrasting changes since 2005, and variations existing across different regions. Regional differences in ozone trends, primarily driven by reductions in NOx and VOC emissions through legislation implemented over the past 30 years, have resulted in significant downward trends in hourly peak O₃ concentrations at most monitoring sites worldwide [17].

3. Exposure to Tropospheric Ozone

3.1. Measuring and Monitoring Tropospheric Ozone

The monitoring and measurement of tropospheric ozone is essential to understanding the dynamics of its formation, distribution, and impacts on human health and the environment. This process involves a combination of ground-based measurement techniques, satellite observations, and air-quality models, each of which provides crucial complementary data for a comprehensive assessment of ozone in the lower atmosphere.

3.1.1. Ground Measurements

Ground measurements are the most direct and detailed method of monitoring ozone concentrations at specific locations. These data are collected through monitoring networks operated by environmental agencies, such as the Environmental Protection Agency (EPA) in the United States (https://www.epa.gov/aqs, accessed on 20 June 2024). Ground-based monitoring stations use instruments, such as chemiluminescence-based ozone analyzers and UV spectrophotometers, to continuously measure ozone concentrations. These devices detect ozone in real time, allowing data to be collected with high temporal and spatial resolutions. For example, the EPA’s air-quality system (AQS) collects detailed data from hundreds of monitoring stations distributed across the country, allowing the analysis of long-term trends and identification of ozone peaks during specific pollution episodes.

3.1.2. Satellite Observations

Satellite observations provide a global view of ozone concentrations, covering large geographical areas that may include remote regions where ground-based monitoring stations are scarce or absent. Satellites, such as the Ozone Monitoring Instrument (OMI) aboard NASA’s Aura satellite, measure ultraviolet and visible radiation reflected from the Earth’s surface and clouds, allowing tropospheric ozone concentrations to be derived [18]. These satellite measurements are particularly useful for monitoring spatial and temporal variations in ozone on a global scale, providing essential data for analyzing the long-range transport of ozone and its precursors. Satellite data, although less detailed than ground-based measurements, offer a wide spatial coverage and can be used to validate and improve air-quality models. A recent investigation employing the Tropospheric Monitoring Instrument (TROPOMI), launched in October 2017 aboard the European Space Agency’s Sentinel-5P, revealed an improved characterization of extreme ground-level ozone events within ozone-concentration modeling frameworks. This success underscores the potential utility of TROPOMI data, in synergy with high-resolution meteorological datasets, to accelerate the development of satellite-driven predictive models for ground-level ozone concentrations [19].

3.1.3. Air-Quality Models

Air-quality models, such as Community Multiscale Air Quality (CMAQ) and GEOS-Chem, are indispensable tools for predicting ozone concentration and understanding the chemical and physical processes that determine its formation and distribution. These models combine observational data with simulations based on chemical and physical equations that describe the interactions between various pollutants and meteorological conditions. Air-quality models can be used to simulate future air-quality scenarios under different emission conditions, helping environmental policy makers to design effective strategies for reducing ozone pollution.

3.1.4. Data Integration

The integration of data from ground-based measurements, satellite observations, and air-quality models enables a more complete understanding of tropospheric ozone dynamics. For example, data from ground-based monitoring stations can be used to calibrate and validate air-quality models, while satellite observations can provide global context and detect transboundary ozone transport phenomena. In addition, the combined use of these different techniques makes it possible to identify the main sources of emissions, assess the effectiveness of pollution control policies, and predict the impacts of climate change on future ozone concentrations.

3.1.5. Challenges and Future Prospects

Despite significant technological advances, tropospheric ozone monitoring still presents several challenges. The spatial resolution of satellite data is often lower than that of ground-based measurements, and measurement techniques can be affected by variables, such as cloud cover and surface reflectivity. Furthermore, air-quality models require accurate input data and a detailed understanding of the chemical and physical processes involved. Future developments could include the use of drones and other mobile platforms to improve measurement coverage and the application of machine learning techniques to improve ozone modeling. Moreover, while reducing ozone precursors (NOx and VOCs) is a primary strategy, the relationship between these pollutants and ozone formation can be complex. Factors, like precursor ratios, sunlight intensity, and meteorology, can influence ozone concentrations. In some situations, an abundance of one precursor can limit ozone formation, even if the other precursor is present. This may be because ozone concentrations are mainly related to VOCs/NOx ratios and the morphologies of VOCs, rather than absolute levels of VOCs and NOx [9]. Finally, ozone is a reactive molecule that can also participate in other chemical reactions.

3.2. Determinants of Exposure

Exposure to ground-level ozone, an air pollutant known for its harmful effects on human health and the environment, is influenced by a number of determinants that include geographical, temporal, behavioral, and demographic variables. Understanding these determinants is crucial to developing effective strategies to mitigate the impact of ozone on the population.

3.2.1. Geographical Factors

Geographical location plays a significant role in ozone exposure. Urban areas, with high traffic densities and industrial activities, tend to have higher ozone levels than rural areas [20], although long-distance transport can also bring ozone and its precursors to remote regions [21]. For example, cities with high volumes of vehicular traffic and numerous industrial emission sources, such as Los Angeles and Beijing, are often subject to high ozone concentrations. In addition, topographical features, such as valleys surrounded by mountains, can trap pollutants and promote ozone formation. Meteorological conditions, such as sunlight intensity and temperature, further influence ozone levels, with peaks during the summer months when photochemical reactions are most active [22].

3.2.2. Temporal Factors

Seasonal and diurnal variations in ozone concentrations are marked [23]. Ozone levels are generally higher in summer due to increased sunlight and temperatures, which accelerate chemical reactions between ozone precursors [24]. During the day, ozone concentrations tend to be highest in the late afternoon, when photochemical reactions have had time to build up. In contrast, at night, ozone levels decrease due to the lack of sunlight and the reaction with nitrogen monoxide (NO) that removes ozone from the atmosphere [25]. In addition, acute pollution episodes, such as heat waves, can cause significant short-term increases in ozone levels [26].

3.2.3. Behavioral Factors

Individual behavioral habits and activities influence personal exposure to ozone. People who spend a lot of time outdoors, such as outdoor workers, athletes, and children playing outdoors, are exposed to higher ozone levels than those who spend more time indoors [27]. This is because, during physical activity, breathing rates increase significantly. As a result, individuals exercising or working outdoors inhale a larger volume of air, and consequently, a greater amount of ozone along with it. Additionally, outdoor environments offer no barrier to ambient ozone, while buildings and ventilation systems in indoor spaces can provide some degree of protection by filtering or diluting outdoor ozone concentrations. Time spent in transport can also influence this, as commuters may be exposed to high concentrations of ozone and other pollutants. In addition, the level of physical activity plays an important role: intense physical activity increases lung ventilation, leading to increased inhalation of ozone and, therefore, an increased risk of adverse health effects [28].

3.2.4. Demographic Factors

Certain demographic groups are more vulnerable to the effects of ozone [29]. Children, the elderly, and people with pre-existing health conditions, such as asthma and cardiovascular disease, are particularly sensitive to ozone exposure [30]. Children are more susceptible because their lungs are still developing and tend to breathe faster than adults, thus inhaling more ozone per unit of time. The elderly, often with a weakened immune system and more likely to have chronic diseases, are also at higher risk [31]. In addition, people with a low socioeconomic status may have higher exposure due to the lack of access to adequately ventilated indoor environments or transport systems that reduce exposure to polluted air [32].

3.2.5. Interactions and Synergies

The interaction between these determinants can amplify overall ozone exposure. For example, an asthmatic child who lives in an urban area with high traffic density and participates in outdoor sports activities during summer is exposed to a significantly higher combined risk than a child who lives in a rural area with less pollution [33]. Moreover, mitigation strategies, such as temporary traffic restrictions during ozone peaks or the promotion of indoor activities on high ozone days, must take these interactions into account to be effective.

4. Health Effects of Tropospheric Ozone

Ground-level ozone is an air pollutant of great concern because of its significant adverse effects on human health [34]. A recent global study estimated that deaths caused by ozone exposure increased significantly by 46% between 2000 and 2019. In 2000, an estimated 290,400 people died from ozone exposure (with a range of 151,800 to 457,600 possible). By 2019, that number jumped to 423,100 (with a range of 223,200 to 659,400 possible). Interestingly, the share of ozone-related deaths in suburban areas (peri-urban areas) stayed around 56% throughout this period. However, there was a shift in urban areas. The percentage of ozone-attributable deaths in cities increased from 35% to 37%, resulting in an additional 54,000 deaths [35]. Hospital admissions for respiratory and cardiovascular diseases were estimated in terms of concentration–response functions expressed as relative risk and 95% confidence interval for short-term ozone exposure and are equal to 1.0044 (1.0007–1.0083) and 1.0089 (1.0050–1.0127), respectively, for respiratory causes and cardiovascular causes [36].

Several epidemiological and toxicological studies have shown that exposure to ozone can cause a wide range of respiratory, cardiovascular, and systemic problems. Health effects can range from acute and transient symptoms to chronic and life-threatening conditions, disproportionately affecting the most vulnerable groups of the population, such as children, the elderly, and individuals with pre-existing health conditions. However, the epidemiological evidence of the association of long-term exposure to ozone is remarkably limited compared to the association of short-term exposure and findings for ozone remained inconsistent.

4.1. Respiratory Problems

One of the most immediate and documented effects of ozone exposure is irritation of the respiratory system. Ozone can irritate the airways, causing symptoms such as coughing, sore throat, and a burning sensation in the chest [37]. These symptoms can occur even at low ozone concentrations and become more severe as exposure levels increase [38]. Ozone induces inflammation in the lungs and can temporarily reduce lung function, making breathing more difficult. Individuals with asthma are particularly susceptible, as ozone can trigger asthma attacks and exacerbate symptoms, often requiring increased use of asthma control medications [39]. Studies have shown that even short-term exposure to ozone can increase the frequency of emergency room visits and hospitalizations for respiratory diseases [40].

4.2. Cardiovascular Effects

Ozone exposure also has significant implications for the cardiovascular system [41]. Several studies have correlated ozone exposure with increased rates of cardiovascular mortality and acute cardiovascular events, such as heart attacks and strokes [42]. Ozone can cause oxidative stress and systemic inflammation, which in turn can contribute to atherosclerosis and other cardiovascular diseases [43]. In addition, ozone can increase blood pressure and alter heart rate variability, two well-known risk factors for heart disease [44]. People with pre-existing cardiovascular disease are particularly at risk, but healthy individuals can also experience adverse effects, especially during periods of high exposure [45].

4.3. Systemic and Long-Term Effects

The effects of ozone are not limited to the respiratory and cardiovascular systems [46]. Prolonged exposure can lead to more serious systemic effects, including damage to the central nervous system and immune system [47,48]. Some studies suggest that chronic exposure to ozone may adversely affect cognitive function and contribute to neurodegenerative diseases, such as Alzheimer’s disease [49], and neurodevelopmental disorders in children, such as attention-deficit/hyperactivity disorder (ADHD) [50]. Researchers found that long-term and short-term ozone exposures have negative and accumulating impacts on sleep quality and might impair brain functioning [51]. A recent cohort study on long-term exposure and mortality from neurological diseases in Canada found positive associations between ozone exposure and mortality due to Parkinson’s, dementia, stroke, and multiple sclerosis [52]. Also, the annual average concentrations of ozone, calculated and linked to individuals by home address, was associated with an increased incidence of depression and anxiety among middle-aged and older adults in UK biobank [53]. Chronic ozone-induced inflammation may also impair the immune system, reducing the body’s ability to fight infections and diseases [54]. In addition, ozone can have harmful effects on reproductive health, with studies indicating an increased risk of complications during pregnancy and preterm birth [55].

4.4. Impacts on Vulnerable Groups

The health effects of ozone are particularly severe for vulnerable groups, including children, the elderly, and people with pre-existing health conditions, such as asthma, heart disease, and diabetes [56]. Children are particularly susceptible because their lungs are still developing and they tend to spend more time outdoors, thus increasing their exposure to ozone [57]. The elderly, often with weakened immune and respiratory systems, are also at higher risk of serious complications from ozone exposure. People with a low socioeconomic status may face higher risks due to a lack of access to adequate medical care and living conditions that reduce ozone exposure [58]. Regarding the lack of access to adequate medical care, regular check-ups with healthcare professionals can help identify potential health issues exacerbated by ozone exposure, allowing for early intervention. Moreover, access to effective medications and treatment plans for respiratory conditions like asthma, which can be worsened by ozone exposure, is crucial. Regarding living conditions, homes in disrepair may have leaky windows and inadequate ventilation systems, allowing more outdoor ozone to infiltrate indoor spaces. Moreover, individuals living in areas with higher overall ozone levels, such as near industrial zones or heavily trafficked roads, may experience greater exposure, even within their homes. Also, during periods of high ozone and hot temperatures, air conditioning can significantly reduce indoor ozone concentrations.

4.5. Mechanisms of Damage

At the cellular level, ozone causes damage through the production of free radicals and oxidative stress. This oxidative stress can damage cell membranes, proteins, and DNA, leading to inflammation and cell death [59]. Ozone-induced inflammation can extend beyond the lungs, affecting the entire organism. The combined effects of oxidative stress and chronic inflammation can contribute to the progression of many chronic diseases, including chronic respiratory, cardiovascular, and neurodegenerative diseases.

4.6. Intervention and Prevention

In order to mitigate the health effects of ozone, individual and policy interventions are required. Individual measures include limiting the amount of time spent outdoors during ozone peaks, using indoor air purifiers, and monitoring air quality through apps and bulletins. At the policy level, strategies include reducing emissions of ozone precursors, such as nitrogen oxides and volatile organic compounds, through stricter regulations on vehicle and industrial emissions, and promoting renewable energy and sustainable transport technologies [60].

5. Policy Implications and Recommendations

The policy implications of ground-level ozone emissions are complex and require an integrated approach that considers both natural and anthropogenic sources of ozone precursors, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). Due to the significant impacts on public health and the environment, it is crucial to develop and implement effective policies to reduce emissions of these precursors and, consequently, ground-level ozone concentrations [61].

5.1. Regulating Industrial and Transport Emissions

One of the main policy recommendations is to reduce NOx and VOC emissions from industrial and transport activities. It is essential that governments establish stricter regulations on vehicle emissions, incentivizing the use of low-emission technologies and promoting the shift to electric vehicles and other forms of sustainable transport [62]. At the same time, industries must be compelled to adopt cleaner technologies and less polluting production processes, such as Industry 4.0 technologies, which are environmentally sustainable, increase manufacturing efficiency, and reduce resource consumption [63]. Regulatory policies should include stricter emission limits, emission control systems, and incentive programs for the adoption of green technologies.

5.2. Promotion of Renewable Energies

The promotion of renewable energy is another crucial aspect. The transition to renewable energy sources, such as wind, solar, and hydropower, can significantly reduce NOx and VOC emissions from burning fossil fuels. Governments should invest in renewable energy infrastructure and offer economic incentives to encourage both businesses and citizens to use clean energy. Favorable tax policies and subsidies for the installation of solar power plants and other renewable technologies can accelerate this transition.

5.3. Improving Urban Air Quality

In urban areas, where ozone concentrations tend to be higher, it is essential to implement specific measures to improve air quality. These measures may include creating low-emission zones, limiting traffic during periods of high ozone concentrations, and promoting public transport. Planting trees and creating urban green spaces can also help reduce ozone levels and improve citizens’ well-being [64].

5.4. Monitoring and Surveillance

Continuous and accurate monitoring of ozone concentrations is essential to better understand the dynamics of its formation and distribution and to assess the effectiveness of emission control policies. Environmental agencies need to invest in both ground-based and satellite-based monitoring networks, integrating collected data with air-quality models [65,66]. This approach makes it possible to quickly identify critical areas and take timely corrective action. Using sensors in citizen science can allow citizens to collect data on ozone concentration, especially in suburban and rural areas with few air-quality monitoring stations. This participatory app roach promotes awareness of air pollution. It is important to note that, while sensor data provide valuable information, they often require calibration and comparison with official instruments to ensure accuracy [67].

5.5. Public Awareness and Education

Educating the public about the risks associated with ground-level ozone and the preventive measures they can take is crucial. Awareness-raising campaigns should inform citizens about periods of high ozone concentration and advise them to limit outdoor activities during these periods. In addition, promoting the use of air-quality apps and bulletins can help people make informed decisions to protect their health. Lastly, ozone biological monitoring methods, which allow us to observe quantitative changes in living organisms physically present in a specific environments, could be applied to citizen science initiatives as well as to worldwide curricula of educational institutions to sensitize the public about ground-level ozone pollution [68].

5.6. International Policies and Collaboration

Since ozone and its precursors can be transported over long distances, international collaboration is crucial [69]. Governments must work together to develop global agreements and regulations to reduce NOx and VOC emissions. International organizations, such as the World Health Organization (WHO) and the United Nations Environment Programme (UNEP), can facilitate the cooperation and the sharing of best practices between countries.

6. Conclusions

Tropospheric ozone emissions and their effects on health highlight a complex dynamic of the formation, distribution, and impact of this pollutant. While stratospheric ozone plays a crucial role in protecting life on Earth by filtering out harmful ultraviolet radiation, tropospheric ozone poses a significant threat to human and environmental health. Ground-level ozone formation is the result of photochemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight, with natural and anthropogenic sources contributing substantially.

Emissions of ozone precursors arise both from natural processes, such as biogenic emissions from forests and soil microbial activities, and from human activities, including transport, industry, and energy production. Ozone formation is influenced by geographical and temporal factors, with higher concentrations in urban and industrialized areas, and during the summer months when solar radiation is most intense. Diesel and gasoline vehicle emissions are a major source of VOCs and NOx in urban environments, necessitating reduction strategies. However, climate change poses a potential complication. It may lead to increased natural emissions of ozone precursors, amplifying ground-level ozone concentrations in pre-existing pollution hotspots, like eastern US, southern Europe, and parts of Asia.

Exposure to tropospheric ozone has well-documented adverse health effects, including respiratory irritation, the exacerbation of pre-existing diseases, such as asthma, and serious implications for the cardiovascular system. Ozone can induce oxidative stress and systemic inflammation, contributing to chronic diseases and potentially long-term neurological damage. Vulnerable groups, such as children, the elderly, and people with pre-existing health conditions, are particularly at risk. Notably, epidemiological data on the association between long-term ozone exposure and health outcomes are scarce compared to short-term exposure studies, resulting in an incomplete understanding of the long-term health effects of ozone. Moreover, most existing studies focus on cardiovascular and respiratory morbidity and mortality, while studies assessing the associations between ozone pollution and neurological diseases are scant.

The measurement and monitoring of tropospheric ozone are crucial to understanding its dynamics and mitigating its effects. Ground-based monitoring methods, satellite observations, and air-quality models provide essential data for a comprehensive assessment and implementation of effective control strategies. Despite the potential of satellite-derived ozone retrievals for ground-level ozone concentration estimation, their application remains limited due to nascent validation efforts and uncertainties in atmospheric correction algorithms.

Policy recommendations to address ground-level ozone include regulating industrial and transport emissions, promoting renewable energy, improving urban air quality, and educating the public on risks and preventive measures. International collaboration is needed to develop and implement global policies to reduce NOx and VOC emissions, given the transboundary nature of ozone transport and its precursors.

In summary, addressing tropospheric ozone requires a multidisciplinary and integrated approach that considers natural and anthropogenic sources, formation and distribution dynamics, health impacts, and mitigation strategies. Only through a comprehensive and coordinated effort will it be possible to effectively reduce tropospheric ozone concentrations and protect human health and the environment.