Air Pollution: What Are Safe Levels?

Air pollution is a complex mixture of solid particles, liquid droplets, and gases that can come from various sources, such as household fuel burning, industrial chimneys, traffic exhausts, and more. It is a significant threat to people worldwide and is associated with millions of premature deaths annually. To address this issue, the World Health Organization (WHO) has developed the Air Quality Guidelines (AQG), which offer global guidance on thresholds and limits for key air pollutants that pose health risks. These guidelines are not legally binding but provide targets for governments to work towards improving air quality and protecting public health. The Air Quality Index (AQI) is a commonly used system that tracks air pollution levels and warns the public when pollution levels become dangerous. The AQI values at or below 100 are generally considered satisfactory, while values above 100 indicate unhealthy air quality.

Air Quality Index (AQI)

The Air Quality Index (AQI) is a system used to inform the public about outdoor air quality and health. It is a yardstick that runs from 0 to 500, with 0 representing perfect air quality and values over 300 indicating hazardous levels of air pollution. The AQI is measured daily and categorised into six colour-coded categories, each corresponding to a range of index values and associated advice. The six categories are:

• Good: AQI value of 50 or below represents good air quality.
• Unhealthy for Sensitive Groups: AQI values above 100 are considered unhealthy, initially for certain sensitive groups of people, including children, the elderly, people with asthma or other lung diseases, and those who are pregnant.
• Unhealthy: As AQI values increase above 100, the air quality becomes unhealthy for everyone.
• Very Unhealthy: Higher AQI values indicate a greater level of health concern for the general public.
• Hazardous: AQI values over 300 represent hazardous air quality, with immediate danger to public health.

The AQI tracks ground-level ozone (a major constituent of smog) and particle pollution (tiny particles from smoke, power plants, factories, vehicle exhaust, and other sources), as well as four other widespread air pollutants. These pollutants include particulate matter (PM), carbon monoxide (CO), nitrogen dioxide (NO2), and volatile organic compounds (VOCs). Newspapers, radio, television, and websites report AQI levels year-round, and maps showing real-time pollution levels are also available to help people protect themselves from the harmful effects of air pollution.

Particulate matter (PM)

PM is categorized based on particle diameter, with particles of 10 micrometers or less (PM10) being inhalable and detrimental to health. PM10 can induce adverse health effects, especially in older adults with chronic heart or lung disease, children, and asthmatics. Fine particulate matter, or particles with a diameter of 2.5 micrometers or less (PM2.5), pose an even greater health risk as they can penetrate deeper into the respiratory tract, reaching the lungs. PM2.5 is associated with short-term health effects such as eye, nose, throat, and lung irritation, coughing, sneezing, and shortness of breath. Long-term exposure to PM2.5 can lead to more serious health issues, including heart disease, asthma, reduced lung function, and lung cancer.

Outdoor sources of PM2.5 include vehicle exhaust, burning wood, gas and other fuels, and fires, including wildfires. Indoor sources include tobacco smoke, cooking, burning candles, fireplaces, and fuel-burning space heaters. Some indoor particles can also originate from biological sources, such as pollen, mold spores, dust mites, and cockroaches.

The World Health Organization (WHO) has developed global air quality guidelines (AQG) to provide thresholds and limits for key air pollutants, including PM. These guidelines offer targets to promote a gradual shift towards lower pollutant concentrations and their associated health benefits. Despite these guidelines, it is estimated that in 2019, 99% of the world's population lived in areas where WHO air quality guidelines were not met, resulting in millions of premature deaths annually.

To monitor air quality and protect public health, the United States Environmental Protection Agency (EPA) uses the Air Quality Index (AQI), which categorizes air quality into six color-coded categories, with higher values indicating greater air pollution and health concerns. An AQI value of 50 or below represents good air quality, while an AQI value over 300 indicates hazardous air quality.

Carbon monoxide (CO)

To prevent CO poisoning, it is crucial to avoid using portable generators or fuel-burning appliances, such as furnaces, stoves, fireplaces, and water heaters, in enclosed or semi-enclosed spaces. These sources can release deadly levels of CO, which can have severe and sometimes fatal consequences. The U.S. Consumer Product Safety Commission (CPSC) recommends installing carbon monoxide alarms on each level of a home outside sleeping areas and conducting annual inspections of fuel-burning appliances to detect potential leaks.

The acceptable levels of CO in the air are outlined by various organisations and regulations. The World Health Organization (WHO) recommends a maximum of 9-10 ppm for no more than 8 hours, 25-35 ppm for no more than 1 hour, and 90-100 ppm for no more than 15 minutes in indoor settings. The Occupational Safety and Health Association (OSHA) sets a personal exposure limit (PEL) of 50 ppm for workers, averaged over an 8-hour period.

In the United States, the Environmental Protection Agency (EPA) establishes national ambient air quality standards (NAAQS) for outdoor air under the Clean Air Act. The NAAQS standard for CO is 9 ppm over 8 hours and 35 ppm over 1 hour, not to be exceeded more than once a year. These standards are designed to protect public health and ensure that CO levels are maintained at safe levels by state, tribal, and local agencies.

Ozone (O3)

Ground-level ozone is not directly emitted into the air but is formed by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). These reactions occur when pollutants from cars, power plants, industrial boilers, refineries, and chemical plants, among other sources, react in the presence of sunlight. Ground-level ozone is a harmful air pollutant and the primary constituent of "smog." It can be transported long distances by wind, affecting both urban and rural areas, and it is most likely to reach unhealthy levels on hot, sunny days.

The harmful effects of ground-level ozone have led to the establishment of National Ambient Air Quality Standards (NAAQS) in the United States. These standards set acceptable ozone levels in the air over specific periods. Counties that fail to meet these standards are designated as "nonattainment" areas, triggering increased pollution control requirements for local businesses. The Environmental Protection Agency (EPA) approves State Implementation Plans (SIPs) that outline regulations to reduce air pollution and improve air quality in these areas.

It is important to note that even short-term exposure to ground-level ozone pollution can have adverse health effects. Research has shown that ozone can harm lung tissue and increase the body's response to other pollutants and allergens. Certain individuals, such as children, the elderly, and people with pre-existing lung diseases, are more vulnerable to the impacts of ozone pollution. Additionally, higher ozone levels, faster breathing rates during outdoor work or exercise, and prolonged outdoor exposure can all increase the health risks associated with ozone inhalation.

While the Clean Air Act and EPA regulations have helped reduce ozone levels in recent decades, climate change and temperature increases continue to contribute to unhealthy ozone levels. Individuals can take steps to protect themselves on days with high air pollution levels and advocate for further air pollution cleanup at various levels of government.

Nitrogen dioxide (NO2)

Outdoor sources of NO2 include motor vehicles such as cars, buses, and trucks, as well as power plants and off-road equipment. NO2 levels are often higher in urban areas due to the higher density of NOx-emitting sources and tend to be higher near busy roads and during peak traffic hours. Indoor sources of NO2 include tobacco smoke and gas-, wood-, oil-, kerosene-, and coal-burning appliances such as stoves, ovens, and heaters, particularly if they are unflued or poorly maintained. The use of gas and solid fuel stoves, especially during meal preparation, can also lead to higher NO2 concentrations.

The World Health Organization (WHO) has set guidelines for indoor air quality, including acceptable levels of NO2. According to the WHO, short-term exposure to concentrations of NO2 higher than 200 µg/m3 can cause inflammation of the airways and may increase susceptibility to respiratory infections. Vulnerable individuals are recommended to take mitigating action when the latest hourly mean concentration exceeds this threshold.

The U.S. Environmental Protection Agency (EPA) has also set national ambient air quality standards (NAAQS) for NO2. The official level of the annual NO2 standard is 0.053 ppm or 53 ppb. The Clean Air Act requires the EPA to periodically review and update these standards to ensure the protection of public health and the environment.

It is important to note that NO2 levels have been declining in many regions due to efforts to reduce emissions. However, certain areas still experience extremely high levels of NO2, which has significant consequences for air quality and public health.

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