Air Pollution: The Most Harmful Toxin Revealed

Air pollution is responsible for 6.7 million premature deaths globally each year, with 99% of people currently breathing air that exceeds the World Health Organization's guideline limits for pollutants. The most common air pollutants include ozone, carbon monoxide, sulfur dioxide, nitrogen oxides, and particulate matter. These substances can cause severe respiratory diseases, heart problems, and environmental degradation such as acid rain and climate change. Of the six pollutants, particle pollution and ground-level ozone are the most widespread health threats. The best way to control air pollution is to transition to cleaner fuels and industrial processes, such as renewable energy sources, maximizing fuel efficiency, and adopting electric vehicles.

Ozone (O3)

Ozone pollution, also known as smog, is considered one of the most dangerous and widespread pollutants in the United States. It can cause serious health issues, especially for children, the elderly, and people with pre-existing lung diseases like asthma. Studies have shown that exposure to ozone irritates and inflames the lungs, leading to increased obstruction of airways and decreased lung function. Long-term exposure to ozone pollution has been linked to respiratory illnesses, metabolic disorders, nervous system issues, and reproductive problems. Additionally, ozone may increase the body's response to other pollutants and allergens.

The impact of ozone pollution is evident in the “State of the Air” reports, which rank cities based on their ozone pollution levels. In 2025, more than 42 million people lived in counties that received an "F" grade for air pollution, with California having the most metro areas on the list. Despite some improvements in certain cities, overall, the number of counties with worsening ozone levels has increased, and the impact of climate change, including wildfires and higher temperatures, has contributed to higher numbers of unhealthy ozone days.

To address the issue of ground-level ozone pollution, the U.S. Environmental Protection Agency (EPA) has designated it as one of the six "criteria" air pollutants, developing limits based on health and environmental criteria. States are working on implementation plans to improve air quality in nonattainment areas, and the EPA is taking actions such as vehicle and transportation standards and regular reviews of air quality standards to reduce emissions of pollutants that form ground-level ozone.

In summary, ozone (O3) is a significant air pollutant that poses a threat to human health and the environment. Its widespread presence, particularly in urban areas, and its ability to cause and exacerbate health issues make it a critical focus for air quality improvement efforts.

Carbon Monoxide (CO)

The harmful effects of carbon monoxide lie in its ability to interfere with oxygen transport in the body. When inhaled, CO reduces the amount of oxygen that can be transported in the bloodstream to vital organs like the heart and brain. This can lead to serious health issues, especially for individuals with heart disease. Exposure to carbon monoxide can cause chest pain, reduced exercise capacity, and other cardiovascular effects. Even healthy individuals can experience adverse effects, such as vision problems, reduced manual dexterity, and difficulty performing complex tasks.

The concentration of CO is crucial in determining its impact on health. Very high levels of CO are typically reached in indoor or enclosed environments and can lead to dizziness, confusion, unconsciousness, and even death. While such extreme levels are unlikely outdoors, elevated outdoor CO levels can still be concerning, especially for people with heart disease.

Regulating CO emissions is essential to mitigate its harmful effects. The U.S. Environmental Protection Agency (EPA) plays a crucial role in setting standards and regulations to control CO pollution and ensure it is maintained at safe levels. These efforts are vital for protecting public health and reducing the impact of CO on the environment.

Additionally, CO contributes to climate change. It participates in chemical reactions in the atmosphere, leading to the formation of ozone, a potent climate change gas. While CO has a weak direct effect on climate change, its indirect contribution through ozone formation is significant. As a result, reducing CO emissions is considered a potential strategy to combat global warming.

Sulfur Dioxide (SO2)

SO2 is harmful to human health and the environment. It can cause a range of adverse effects on the lungs, including wheezing, shortness of breath, and chest tightness, especially during exercise or physical activity. Long-term exposure to high levels of SO2 increases respiratory symptoms and reduces lung function. Short exposures to peak levels of SO2 can also make it difficult for people with asthma to breathe during outdoor activities.

Additionally, SO2 contributes to acid rain, which damages the environment and sensitive ecosystems. It can react with other compounds in the atmosphere to form fine particles that reduce visibility (haze) and stain and damage stone and other materials, including culturally important objects. SO2 and other sulfur oxides can also harm trees and plants by damaging foliage and decreasing growth.

To address the issues caused by SO2, the U.S. Environmental Protection Agency (EPA) has implemented national and regional rules to reduce SO2 emissions and other pollutants that form sulfur oxides (SOx). These regulations are designed to protect human health and the environment by improving air quality and reducing the harmful impacts of SO2 pollution.

Nitrogen Oxides (NOx)

In urban areas, the primary sources of nitrogen oxides are vehicles, power plants, and off-road equipment that burn fuel. The combustion of gasoline in automobiles, as well as coal, oil, and gas, releases nitrogen oxides into the atmosphere. Road traffic is the principal outdoor source of nitrogen oxides, particularly in dense urban areas. The ambient concentration of nitrogen oxides can exceed a total of 500 μg/m3 in these densely populated regions.

Nitrogen dioxide (NO2) is the most common nitrogen oxide and is considered the primary pollutant. It is a highly reactive gas with a foul smell. High concentrations of NO2 can irritate and cause inflammation in the airways of the human respiratory system. Short-term exposure can aggravate respiratory diseases, especially asthma, leading to symptoms like coughing, wheezing, and difficulty breathing. Prolonged exposure to elevated levels of NO2 may contribute to the development of asthma and increase susceptibility to respiratory infections. Vulnerable groups, including people with asthma, children, and the elderly, are at greater risk of adverse health effects from NO2.

Nitrogen oxides can also react with other chemicals in the air to form secondary pollutants, such as ozone, particulate matter, acid rain, and other toxic substances. When NOx reacts with volatile organic compounds (VOCs) in the presence of sunlight, it forms ozone, a major secondary pollutant. Both ozone and particulate matter are harmful to the respiratory system when inhaled. Additionally, high levels of NOx can negatively impact vegetation, making plants more susceptible to diseases and frost damage.

Particulate Matter (PM)

PM is categorised based on the diameter of the particles. PM2.5 refers to fine inhalable particles with diameters of 2.5 micrometers or smaller, which is about 30 times smaller than the diameter of a human hair. These fine particles pose the greatest risk to health as they can penetrate deep into the lungs and even enter the bloodstream. They are the main cause of reduced visibility (haze) in certain regions and have been linked to adverse health effects such as reduced lung function growth in children and increased hospitalisations for cardiovascular and respiratory diseases.

PM10 refers to particles with diameters of 10 micrometers or less, which are still small enough to be inhaled into the lungs and cause adverse health effects. While the specific health impacts of long-term PM10 exposure are less clear, studies suggest a potential link to respiratory mortality.

Particulate matter originates from both outdoor and indoor sources. Outdoor particles are emitted from construction sites, unpaved roads, fields, smokestacks, fires, and complex reactions of other pollutants in the atmosphere. Indoor sources include biological allergens like pollen, mould spores, and cockroaches, as well as activities such as smoking, cooking, burning candles or incense, and using household cleaning products.

To protect public health, the EPA has established regulations and standards for particulate matter pollution. The Air Quality Index (AQI) is used to inform the public about the air quality in their region and any associated health risks, helping individuals take necessary precautions. Additionally, the EPA works with state and local governments to implement rules that reduce emissions of pollutants that contribute to PM, ultimately improving air quality.

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