Visible Air Pollution: What's Harming Us The Most?

Air pollution is a pressing issue that poses a significant threat to both human health and the planet. It refers to the release of pollutants into the air, which can have detrimental effects on the natural environment and human well-being. While some pollutants are invisible, there are visible forms of air pollution that are equally harmful. One of the most visible types of air pollution is smog, which forms when emissions from combusting fossil fuels interact with sunlight. This phenomenon is particularly noticeable in cities, where it can irritate the eyes and throat and cause or exacerbate respiratory issues. Smog is a secondary pollutant, formed through the interaction of primary pollutants such as sulfur dioxide from factories. Other sources of smog include cars, trucks, power plants, and anything that combusts fossil fuels. Recognizing the presence of both visible and invisible air pollutants is crucial in addressing the impact of air pollution on our planet and public health.

Smog: a mixture of smoke and fog, often found in cities

Smog, a visible form of air pollution, is a combination of smoke and fog. The term "smog" was first used in 1905 by Henry Antoine Des Voeux in his paper, "Fog and Smoke," presented at a meeting of the Public Health Congress. Des Voeux observed that large cities, particularly London, had a unique atmosphere of smoky fog, which he dubbed "smog."

Smog forms when emissions from burning fossil fuels, such as coal, gasoline, or natural gas, react with sunlight. It is a secondary air pollutant, created through the interaction and reaction of primary pollutants. While smog can form in any climate with significant industrial or urban pollution, it is more prevalent in warmer, sunnier weather. This is because the upper air temperature inhibits vertical circulation, allowing smog to build up to dangerous levels.

Smog is particularly harmful to human health, causing various respiratory issues and eye and throat irritation. It can also worsen existing conditions like asthma and allergies, triggering asthma attacks. The pollutants in smog can penetrate the lungs and bloodstream, exacerbating bronchitis and increasing the risk of heart attacks.

Cities in northern India, such as Delhi, have been grappling with severe winter smog in recent years. Similarly, Chinese cities sometimes have to close down roads, schools, or airports due to smoke from coal-fired heating during autumn and winter. Mexico City, due to its geographical location, also experiences significant smog, trapping industrial and vehicle pollution.

Reducing smog requires mitigating the primary sources of air pollution, such as the burning of fossil fuels and the emissions from vehicles, factories, and power plants. Recognizing the health and environmental impacts of smog is crucial to drive the necessary changes to reduce air pollution and improve air quality, especially in densely populated urban areas.

Soot: tiny particles of chemicals, soil, smoke, dust, or allergens

Air pollution is a leading cause of environmental changes and a significant risk factor for early death. It is responsible for nearly seven million deaths globally each year, with 4.5 million linked to outdoor air pollution and 2.2 million caused by indoor air pollution.

One of the most visible types of air pollution is soot, which consists of tiny particles of chemicals, soil, smoke, dust, or allergens that are carried in the air. Soot is a type of particulate matter known as PM2.5, with particles no more than 2.5 microns wide—30 times smaller than the width of a human hair. These particles are easily inhaled and can become embedded in our lungs, facilitating the transport of toxic compounds into the bloodstream.

Soot is formed by the incomplete combustion of fossil fuels, wood, waste, and other biomass fuels. Sources of soot include coal-fired power plants, industrial facilities, combustion engines, wildfires, agriculture, vehicle tires, road dust, and volcanic eruptions. In Asia and Africa, residential solid fuels contribute 60-80% of soot emissions, while in Europe and North America, diesel engines are responsible for about 70% of emissions.

The health impacts of soot pollution are severe. It has been linked to increased infant mortality, hospital admissions for heart and lung diseases, cancer, asthma severity, respiratory illness, impaired immune health, metabolic disorders, nervous system impacts, and more. Soot is particularly harmful to vulnerable communities, including children, older adults, and people with pre-existing health conditions.

To address the dangers of soot pollution, the United States Environmental Protection Agency (EPA) has implemented stronger standards and protections. In 2024, the EPA finalized a new standard of 9 micrograms of soot per cubic meter, a significant improvement from the previous level of 12. This regulation aims to improve air quality and protect the health of families, workers, and communities, especially those disproportionately impacted by particle pollution.

Carbon monoxide: a colourless, odourless gas, harmful to humans

Air pollution is the fourth-largest risk factor for premature death globally, with 4.5 million deaths linked to outdoor air pollution and another 2.2 million caused by indoor air pollution in 2019. It is defined as the release of pollutants into the air, which are detrimental to human health and the planet.

One of the most harmful pollutants is carbon monoxide (CO), a colourless, odourless, and deadly gas. It is produced by the incomplete combustion of carbon-containing fuels such as natural gas, gasoline, or wood. CO is emitted by a wide range of combustion sources, including motor vehicles, power plants, wildfires, and incinerators. The majority of outdoor CO emissions, especially in urban areas, come from mobile sources such as cars and trucks.

The danger of carbon monoxide lies in its ability to inhibit the blood's capacity to transport oxygen to the body's organs. CO binds to haemoglobin in the blood, reducing oxygen delivery. This can lead to severe health issues and even death, especially in vulnerable individuals such as unborn babies, infants, the elderly, and people with anaemia or cardiovascular and respiratory diseases. Common symptoms of CO exposure include fatigue, headaches, confusion, and dizziness. Prolonged exposure to high levels of CO can result in oxygen deprivation and death.

Carbon monoxide is often referred to as the "silent killer" due to its invisible and odourless nature. Its presence is often undetected, making it even more hazardous. During the cold season, cases of CO poisoning tend to increase due to the use of improperly vented space heaters and gas ranges for indoor heating. Therefore, it is crucial to have proper ventilation systems, especially when using fuel-burning equipment like generators, stoves, heaters, and grills, which can contribute to dangerous carbon monoxide levels in enclosed spaces.

Particulate matter: PM2.5 and PM10 refer to the size of particles

Air pollution is typically classified as either visible or invisible. Particulate matter (PM) refers to a mixture of solid particles and liquid droplets found in the air. PM is classified by particle size, with PM10 referring to particles 10 micrometres or less in diameter, and PM2.5 referring to particles 2.5 micrometres or less in diameter. PM2.5 is a subset of PM10, and both are easily carried by the wind. PM10 can be inhaled into the lungs and can have adverse health effects, while PM2.5 poses a greater risk to health and is the main cause of reduced visibility in some regions.

PM10 and PM2.5 are produced by a wide range of industrial processes, including combustion and fossil fuel power plants. They can also be generated by natural sources such as bushfires, dust storms, pollen, and sea spray. PM10 is composed of dust from construction sites, landfills, agriculture, wildfires, and industrial sources, while PM2.5 is primarily produced by the combustion of gasoline, oil, diesel fuel, or wood.

The health effects of PM10 and PM2.5 include toxic effects through the absorption of toxic materials into the blood, allergic reactions, bacterial and fungal infections, and increased respiratory symptoms. PM2.5 is more likely to reach the deeper parts of the lung, while PM10 tends to deposit in the upper region of the lung. Short-term exposure to PM2.5 has been linked to premature mortality, increased hospital admissions for heart or lung issues, acute and chronic bronchitis, asthma attacks, and emergency room visits.

While PM10 and PM2.5 refer to particle size, it is important to note that particulate matter is not defined by its chemical composition. Instead, it is a classification of particles by size, and its effects depend on its composition, concentration, and the presence of other pollutants. Particulate matter can affect the aesthetics and utility of areas through visibility reduction and can impact buildings and vegetation.

Nitrogen dioxide: a harmful gas, especially to those with asthma or allergies

Air pollution is a pressing issue that poses significant risks to human health and the planet. While many pollutants are invisible, some forms of air pollution are visible, such as smog and soot. Smog, or ground-level ozone, occurs when emissions from burning fossil fuels interact with sunlight, while soot consists of tiny particles of chemicals, soil, smoke, dust, or allergens suspended in the air.

Among the various air pollutants, nitrogen dioxide (NO2) stands out as a highly reactive gas that poses considerable harm, especially to individuals with asthma or allergies. NO2 is one of the six widespread air pollutants with national air quality standards in place to limit their levels in outdoor air. It primarily enters the air through the burning of fuels, with significant sources including cars, trucks, buses, power plants, and industrial processes.

The health risks associated with NO2 exposure are well-documented. Short-term exposure can irritate the airways and aggravate respiratory diseases, particularly asthma, leading to coughing, wheezing, and difficulty breathing. Prolonged exposure to elevated NO2 concentrations may even contribute to the development of asthma and increase susceptibility to respiratory infections. Scientific evidence also suggests a link between NO2 exposure and adverse effects on the heart and lungs, pregnancy and birth outcomes, and a potentially higher risk of kidney and neurological harm, autoimmune disorders, and cancer.

Individuals with pre-existing medical conditions, such as asthma, chronic obstructive pulmonary disease (COPD), cardiovascular disease, and diabetes, are at an elevated risk from NO2 pollution. Those living near emission sources, such as major roadways or industrial sites, are also more vulnerable to its harmful effects. Additionally, infants and children are particularly susceptible due to their higher breathing rate relative to their body weight and greater duration of outdoor exposure.

To address the issue of NO2 pollution, regulatory measures like the federal Clean Air Act have been implemented, leading to a decline in nitrogen dioxide emissions. Power plants, industrial sites, and on-road vehicles have become cleaner, contributing to improved air quality nationwide. However, it is essential to continue advocating for the cleanup of air pollution and supporting research and education initiatives to protect human health and the environment from the detrimental impacts of nitrogen dioxide and other air pollutants.

Frequently asked questions