Lingering Effects: Understanding Air Pollutants' Residual Activity

Air pollution is the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere. It is caused by the release of pollutants that are detrimental to human health and the planet as a whole. These pollutants can be solids, liquids, or gases, and they are typically produced in higher-than-usual concentrations, reducing the quality of the environment. While the sources of air pollution are multiple and context-specific, they often include human activities such as the combustion of fossil fuels, industrial processes, transportation, and household activities. The residual activity of air pollutants refers to the lasting impacts and effects of these pollutants on the environment, climate, and human health, even after the initial release or emission.

Particulate matter (PM)

The health risks associated with PM are well documented. PM can penetrate the respiratory system via inhalation, causing respiratory and cardiovascular diseases, reproductive and central nervous system dysfunctions, and cancer. Long-term exposure to PM has been linked to adverse perinatal outcomes and lung cancer. In 2013, the WHO's International Agency for Research on Cancer (IARC) classified PM as a cause of lung cancer. PM is also capable of penetrating deep into the lungs and entering the bloodstream, causing cardiovascular (ischaemic heart disease), cerebrovascular (stroke), and respiratory impacts. Both long-term and short-term exposure to PM is associated with morbidity and mortality from cardiovascular and respiratory diseases.

The major constituents of PM are residual solid carbon and inorganic compounds such as alkali and alkaline earth metals, silica, and iron. The minor constituents are arsenic, selenium, antimony, zinc, and lead. The largest particles, known as coarse particles, have diameters between 2.5 and 10 micrometres and are sourced from pollen, sea spray, and wind-blown dust from erosion, agricultural spaces, roadways, and mining operations. The finer particles, known as PM2.5, have diameters of 2.5 micrometres or less and are derived from primary sources such as the combustion of fuels in power generation facilities, industries, or vehicles, and secondary sources such as chemical reactions between gases.

PM2.5 is considered to pose the greatest risk to health. It is the main cause of reduced visibility (haze) in parts of the United States and other countries. The EPA in the US has set an air quality standard for PM2.5 of 65 µg m-3, 24-hour mean. The WHO has recommended that PM2.5 be routinely monitored due to its association with health effects. However, the WHO also acknowledges the toxicological activity of the coarse fraction of PM10, and thus recommends that PM10 measurements continue.

To protect against the harmful effects of PM, organisations such as the EPA and the WHO provide air quality alerts and guidelines. The Air Quality Index (AQI) is used to inform the public about the level of air pollution and associated health risks.

Carbon monoxide (CO)

The molecular weight of carbon monoxide is 28.01 g/mol, similar to that of air (approximately 29). It has a melting point of −205.1 °C and a boiling point of −191.5 °C at 760 mmHg. Its density is 1.250 kg/m3 at 0 °C and 1 atm, and its solubility in water at 1 atm is 3.54 ml/100 ml at the same temperature. These physical properties allow it to mix freely with air in any proportion and move with air via bulk transport. It is also combustible and can form explosive mixtures with air.

Carbon monoxide is dangerous because it reduces the oxygen-carrying capacity of the blood, leading to hypoxia. This can cause a variety of health issues, including transient loss of consciousness, cognitive impairments, and residual coordinative deficits. In extreme cases, carbon monoxide poisoning can be fatal. The health effects of carbon monoxide exposure depend on the concentration and duration of exposure, as well as individual characteristics and activities.

Indoor sources of carbon monoxide include faulty gas heaters, open fires, and inefficient stoves or space heaters. Outdoor sources include vehicle emissions, industrial activities, power plants, and waste burning. Policies and interventions that promote cleaner energy sources, improved combustion technologies, and better ventilation can help reduce carbon monoxide levels and mitigate the health risks associated with this pollutant.

Carbon monoxide also has some physiological importance. It is produced during the degradation of heme, a component of red blood cells, to bilirubin. This gaseous molecule plays a role in neuronal signaling and the modulation of vascular tone. However, it is also involved in some pathological conditions such as ischemia.

Ozone (O3)

Ozone in the stratosphere forms the "ozone layer", which is approximately 6 to 30 miles above the Earth's surface and acts as a protective barrier against harmful UV radiation. This “good” ozone is beneficial to life on Earth and plays a protective role against harmful ultraviolet irradiation.

However, ground-level or tropospheric ozone is considered "bad" due to its negative impact on human health and the environment. Ground-level ozone is not directly emitted into the air but is formed by chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs). These reactions traditionally depend on the presence of heat and sunlight, resulting in higher ambient ozone concentrations during the summer months. However, it is important to note that high ozone concentrations can also occur during colder months in specific circumstances, such as in high-elevation areas with significant VOC and NOx emissions, snow cover, and near or below-freezing temperatures.

Tropospheric ozone contributes to smog formation and can be transported long distances by wind, affecting both urban and rural areas. It is a major air pollutant and a significant component of smog, which is composed of photochemical reactions with pollutants emitted from vehicles and industrial activities. When inhaled, ground-level ozone reacts chemically with biological molecules in the respiratory tract, leading to adverse health effects, particularly for individuals with asthma or other respiratory conditions.

The presence of residual ozone has been studied in the context of wastewater treatment. Research suggests that residual ozone can enhance the removal of organics and shape the microbial community. For example, in the treatment of petrochemical wastewater, residual ozone has demonstrated relatively high chemical oxygen demand (COD) removal efficiency. Additionally, ozone pretreatment has been found to disrupt the molecular chemical structure of pollutants, improving the biochemical properties of biofilms and benefiting subsequent treatment processes.

Nitrogen dioxide (NO2)

NO2 is a major air pollutant and a serious public health concern. According to the World Health Organization (WHO), almost the entire global population (99%) breathes air that exceeds the WHO guideline limits and contains high levels of pollutants, with low- and middle-income countries suffering the highest exposures. Short-term exposure to high concentrations of NO2 can irritate the airways and aggravate respiratory diseases, particularly asthma. It can cause coughing, wheezing, and difficulty breathing, and may increase susceptibility to respiratory infections. Longer exposures to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase the risk of respiratory infections.

In addition to its harmful effects on human health, NO2 can also cause environmental damage. It interacts with water, oxygen, and other chemicals in the atmosphere to form acid rain, which harms sensitive ecosystems such as lakes and forests. Deposition of chemical compounds containing NOx can increase nitrogen concentrations in soil or water, leading to significant long-term harm to habitats.

Efforts to reduce NO2 emissions have been made in recent years, particularly in the UK. Between 1992 and 2002, the annual mean NO2 concentration at urban background sites decreased by an average of 2.7 µg/m3 each year. This trend continued between 2006 and 2019, with a further reduction of 0.9 µg/m3 per year. In 2020, the annual mean NO2 concentration decreased by 4.5 µg/m3 compared to 2019, likely due to reduced traffic during COVID-19 restrictions. However, concentrations increased by 5% in 2021 as restrictions were lifted. The implementation of stricter emission standards for road vehicles and the move away from coal power generation have contributed to these positive trends.

Household sources of NO2 include the use of unvented gas stoves, combustion space heaters, and wood-burning appliances. A study by Levy et al. examined nitrogen dioxide concentrations in homes across 15 countries, finding that the use of gas stoves and combustion space heaters significantly influenced indoor NO2 levels. In developing countries, the combustion of biomass, such as wood, crop residues, and animal dung for cooking and heating, can also contribute to elevated NO2 concentrations.

Sulfur dioxide (SO2)

Air pollution is the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere. It is caused by human activities that adversely affect the environment by polluting the air we breathe, the water we drink, and the soil in which plants grow. Air pollution poses a major threat to both human health and the climate.

SO2 is harmful to both human health and the environment. It can affect human health by penetrating deep into the lungs and contributing to respiratory issues. In fact, the Clean Air Act in the US identifies sulfur dioxide as one of the six pollutants that are harmful to public health and the environment, along with ozone, particulate matter, nitrogen dioxide, carbon monoxide, and lead.

Additionally, SO2 and other sulfur oxides (SOx) contribute to the formation of acid rain, which can harm sensitive ecosystems. They can also react with other compounds in the atmosphere to form fine particles that reduce visibility (haze) and damage stone and other materials, including culturally important objects. High concentrations of gaseous SOx can also harm trees and plants by damaging foliage and decreasing growth.

To mitigate the harmful effects of SO2, the US Environmental Protection Agency (EPA) has established national and regional rules to reduce SO2 emissions and other pollutants that form sulfur oxides. These regulations help state and local governments meet the Agency's national air quality standards and protect public health and the environment.

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