Air Pollution Indicators: What You Should Know

Air pollution is a pressing issue that affects billions of people worldwide, causing an estimated 6.7 million premature deaths annually. It is caused by the contamination of the indoor or outdoor environment by chemical, physical, or biological agents that alter the natural composition of the atmosphere. Common sources of air pollution include household combustion devices, motor vehicles, industrial facilities, and forest fires. Lichens, which are complex life forms composed of fungi and algae, are widely used as bio-indicators of air pollution, particularly for sulfur dioxide. They are sensitive indicators that absorb pollutants from their surroundings, providing valuable information about the environment. Another indicator of air pollution is the Air Quality Index (AQI), a tool used by the EPA to communicate outdoor air quality and its associated health risks. This indicator categorizes air quality into six color-coded levels, with higher values representing increased pollution and health concerns. Particulate matter (PM), composed of sulfates, nitrates, ammonia, sodium chloride, black carbon, mineral dust, and water, is also a significant indicator of air pollution and is linked to various health issues.

Lichens are bio-indicators

Lichens are widely used as environmental indicators or bio-indicators. They are a complex life form that is a symbiotic partnership of two separate organisms: a fungus and an alga. The fungus is usually an Ascomycetes, and the alga can be either a green alga or a blue-green alga (cyanobacteria), with many lichens containing both types of algae. The fungus provides shelter and water, while the alga provides nutrients, formed through chlorophyll.

Lichens are well-known as sensitive indicators of air pollution, particularly sulfur dioxide. They directly benefit humans by absorbing everything in their atmosphere, especially pollutants. Lichens can provide us with valuable information about the environment around us. Any heavy metals, carbon, sulfur, or other pollutants in the atmosphere are absorbed into the lichen thallus.

Lichens are also sensitive to nitrogen (N) levels in the atmosphere. Nitrogen deposition can increase the load of nutrients, and too much nitrogen can harm and kill the algae's chlorophyll, which is used to produce sugars that feed the lichen and the fungus. Scientists monitor lichen communities, and an increase in nitrogen-tolerant species, along with a decrease in nitrogen-sensitive species, can indicate rising nitrogen deposition. This shift in species composition can exemplify the potential beginning of ecosystem decline due to nitrogen deposition.

Lichen monitoring has been used in environmental impact studies, particularly in Italy, to evaluate the quality of the air in the area surrounding an industrial plant. The data collected from monitoring lichen health can be paired with atmospheric deposition data to determine the sources and levels of pollution causing detrimental effects. Lichens are also used to map out the quality of the air in a determined area, with the number, frequency, and tolerance of lichens present in an area providing an evaluation of the level of atmospheric pollution.

Lichens have also been identified as significant contributors to particulate matter trapping, which is a major proxy indicator for air pollution. Their gradual growth rate allows them to accumulate substances, and they exhibit exceptional nutrient cycling, contributing to ecosystems from a local to a regional scale.

Particulate matter (PM)

PM is classified based on particle size, with PM2.5 and PM10 being the most common in regulatory frameworks. PM2.5 refers to particles with diameters of 2.5 micrometres or less, while PM10 includes particles up to 10 micrometres. These particles are so small that they can be inhaled, penetrating deep into the lungs and even entering the bloodstream. This can lead to serious health issues, including respiratory and cardiovascular problems. Long-term exposure to PM has been linked to lung cancer, adverse perinatal outcomes, and increased mortality, especially for those with pre-existing heart or lung conditions.

The sources of PM vary in size and shape and can be composed of hundreds of different chemicals. Primary sources of PM2.5 include the combustion of fuels in power generation, industries, and vehicles, while secondary sources include chemical reactions between gases. Coarse particles, with diameters between 2.5 and 10 micrometres, originate from pollen, sea spray, and wind-blown dust from erosion, agriculture, roadways, and mining.

Reducing PM emissions is crucial for protecting public health. The US Environmental Protection Agency (EPA) has implemented rules to reduce pollutants that form PM, helping state and local governments meet national air quality standards. Additionally, the World Health Organization (WHO) supports countries in developing evidence-based policies and interventions to tackle air pollution and reduce its health impacts.

To protect oneself from harmful levels of PM, individuals can utilise air quality alerts and notifications provided by organisations like AirNow. These alerts are based on the Air Quality Index (AQI), which translates air quality data into understandable information, helping people take appropriate actions to safeguard their health.

Carbon monoxide (CO)

CO is a harmful pollutant that can have detrimental effects on human health when inhaled in large amounts. It reduces the amount of oxygen transported in the bloodstream to vital organs, including the heart and brain. Elevated levels of CO, particularly in enclosed spaces, can lead to dizziness, confusion, unconsciousness, and even death. According to the California Air Resources Board, over 400 people in the United States die each year from CO poisoning.

Outdoor air pollution, including CO emissions, is a pressing environmental health issue worldwide. In 2019, it was estimated that 4.2 million premature deaths occurred globally due to outdoor air pollution, with 89% of these deaths happening in low- and middle-income countries. While very high levels of CO are unlikely to occur outdoors, elevated outdoor CO levels can be concerning for individuals with heart disease.

Indoor CO levels can be significantly higher than outdoors due to various sources, such as gas stoves, malfunctioning or improperly vented gas appliances, space heaters, fireplaces, tobacco smoke, and car emissions. During the cold season, CO poisoning cases tend to increase due to the increased use of improperly vented space heaters and gas ranges for heating.

To mitigate the health risks associated with CO, the U.S. Environmental Protection Agency (EPA) sets and reviews standards for CO levels in outdoor air under the Clean Air Act. These standards help state, tribal, and local agencies ensure that CO is maintained at safe levels. Additionally, CO emission reductions are considered a potential strategy to mitigate the effects of global warming, as CO contributes to climate change through chemical reactions in the atmosphere that produce ozone.

Nitrogen dioxide (NO2)

NO2 is a major air pollutant in urban areas, with the highest concentrations found in large cities and on or near major roadways. In North America, NO2 is primarily produced from burning fossil fuels for transportation, power generation, and industrial activity, as well as from wildfires. Trucks, buses, and cars are the largest sources of NO2 emissions, followed by diesel-powered non-road equipment and industrial processes.

NO2 is harmful to both human and ecosystem health. It can cause a range of negative health effects, particularly to the lungs. People who live near emission sources are at a higher risk of experiencing the health impacts of NO2 pollution. Other vulnerable subpopulations include those with pre-existing medical conditions such as asthma, chronic obstructive pulmonary disease (COPD), cardiovascular disease, diabetes, and lung cancer. Scientific evidence suggests that exposure to NO2 may cause asthma in children. Long-term exposure to NO2 has been linked to adverse health outcomes, including an increased risk of morbidity and mortality from cardiovascular and respiratory diseases.

To monitor and address NO2 pollution, organizations like the WHO and NASA have taken several measures. The WHO's Air Quality and Health Unit works to reduce air pollution levels and protect populations from health risks through knowledge, evidence, and progress measurement. NASA's Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument collects measurements of nitrogen dioxide air pollution over North America, providing valuable data on the amounts of NO2 in the atmosphere. These efforts contribute to the ongoing work of reducing NO2 emissions and improving air quality worldwide.

Outdoor and indoor air pollution

Outdoor air pollution is a major environmental health problem affecting people in low-, middle-, and high-income countries. Outdoor air pollution is primarily a result of emissions from various sources, including factories, fossil fuels, cars, and more, depending on the area. The combustion of fossil fuels is a well-known source of outdoor air pollution. For example, smoke from factories or the burning of wood, and toxic gases released from coal, are all indicators of outdoor air pollution.

In 2019, outdoor air pollution was estimated to have caused 4.2 million premature deaths worldwide. Most sources of outdoor air pollution are beyond the control of individuals, and this demands action by local, national, and regional policymakers. Outdoor air pollution in both cities and rural areas contributed to these deaths, mainly due to exposure to ischaemic heart disease and stroke, chronic obstructive pulmonary disease, acute lower respiratory infections, and lung cancers.

Particulate matter (PM) is a common proxy indicator for air pollution. PM10 and PM2.5 refer to particulate matter smaller than 10 and 2.5 microns in diameter, respectively. These particles can penetrate deep into the lungs and enter the bloodstream, causing cardiovascular and respiratory issues. PM2.5 is mostly produced by human activity, such as vehicle exhaust, factory emissions, and smoke from burning wood and biomass fuels.

Indoor air pollution, on the other hand, refers to the air quality contamination in buildings caused by dust, dirt, and other pollutants. People generally spend more than 90% of their time indoors, and indoor air pollution levels can be 2 to 5 times higher than outdoor pollution levels. Inadequate ventilation can increase indoor pollution levels, as can high temperatures and humidity. Various volatile organic compounds (VOCs) are released from cleaning products, paints, beauty products, and disinfectants, causing eye and throat inflammation and severe migraines. Additionally, toxic gases can accumulate inside buildings, and chemicals in furniture, electronics, and building materials can contribute to indoor air pollution.

While outdoor air pollution is a serious issue, it is important to recognize that indoor air pollution can be even more harmful due to the amount of time spent indoors and the potential for pollutant concentrations to build up. Outdoor air pollutants can infiltrate indoor spaces through open windows and doors, ventilation systems, and cracks in walls and doors. Therefore, addressing both outdoor and indoor air pollution is crucial to protecting public health.

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