Understanding Air Quality Index: Calculating Clear Air

The Air Quality Index (AQI) is a representation of air pollution concentration levels, developed by the EPA to provide a simple, uniform way to report daily air quality conditions to the general public. The AQI is calculated by converting measured pollutant concentrations to a uniform index based on the health effects associated with a pollutant. The pollutants measured include ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and two sizes of particulate matter.

Calculating AQI from pollutant concentration

The Air Quality Index (AQI) is a representation of air pollution concentration levels. It was developed by the Environmental Protection Agency (EPA) to provide a simple, uniform way to report daily air quality conditions to the general public. The AQI is calculated by converting measured pollutant concentrations to a uniform index based on the health effects associated with a pollutant. The health benchmarks used for calculating the AQI are pollutant-specific and are established by the EPA through the National Ambient Air Quality Standards. The Clean Air Act requires the EPA to review these standards every five years.

The AQI includes measures for six major air pollutants: ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and two sizes of particulate matter (PM 10 and PM 2.5). PM 10 particles have a size of less than or equal to 10 microns, while PM 2.5 particles are ultra-fine and have a size of less than or equal to 2.5 microns. These particles can be released from various sources, including construction, smoking, cleaning, renovations, demolitions, natural hazards, and industrial emissions. When inhaled, they can penetrate deep into the respiratory system and cause respiratory issues such as asthma, coughing, and irritation in the airways, eyes, and nose.

To calculate the AQI, data for a minimum of three pollutants must be present, with one being either PM10 or PM2.5. The AQI ranging from 0-500 has different concentrations for each pollutant and corresponding health effects. For example, a range of 0-50 indicates good air quality, with minimal or no impact on health. Readings between 50 and 100 are considered moderate and may affect unusually sensitive individuals. When the AQI exceeds 100, air quality is considered unhealthy for the public. Levels above 300 are rare in the United States.

It is important to note that AQI forecasts heavily depend on temperatures, precipitation, wind, and cloud cover. Weather conditions can increase the rate of air pollutant formation or accumulation. For instance, ozone is formed by a chemical reaction between nitrogen oxides and volatile organic compounds, and hot and sunny weather accelerates this process. Fine particle pollution can increase under specific conditions such as high humidity or pressure, strong overnight temperature inversions, or low wind speeds.

AQI benchmarks and health effects

The Air Quality Index (AQI) is divided into six colour-coded categories, each corresponding to a different level of health concern and ranging from good to hazardous air quality. Each category has a specific colour, making it easy for people to quickly determine whether the air quality is reaching unhealthy levels in their communities. The six categories are: Good, Moderate, Unhealthy for Sensitive Groups (USG), Unhealthy, Very Unhealthy, and Hazardous.

An AQI value of 100 generally corresponds to the level of the short-term National Ambient Air Quality Standard. AQI values at and below 100 are generally considered to be satisfactory. When AQI values are above 100, air quality is considered unhealthy, at first for certain sensitive groups of people, then for the entire population as AQI values increase. For example, when the AQI value for particle pollution is between 101 and 150, or Code Orange, air quality is considered “unhealthy for sensitive groups”. In this range, people with heart or lung disease, older adults, children, people with diabetes, and people of lower SES are advised to reduce prolonged or heavy exertion.

The Clean Air Act requires the EPA to set National Ambient Air Quality Standards for pollutants considered harmful to public health and the environment. The EPA has set National Ambient Air Quality Standards for six criteria pollutants: sulfur dioxide (SO2), particulates (PM2.5/PM10), nitrogen oxides (NOx), carbon monoxide (CO), ozone (O3), and lead (Pb). These standards are periodically reviewed and may be revised.

Evidence indicates that air quality alerts are helpful and that advocacy from healthcare professionals can lead to positive behaviour changes. For example, a survey in Kansas showed that people were almost four times as likely to change their outdoor activity levels if they were advised to do so by a healthcare professional.

Weather conditions and pollution formation

Weather conditions play a significant role in the formation and dispersion of pollution, influencing air quality. Meteorological parameters such as temperature, solar radiation, wind speed, and humidity interact with pollution in complex ways.

High temperatures and solar radiation, for example, promote the dispersion of pollutants both vertically and horizontally. However, they also contribute to the formation of photochemical ozone, a harmful pollutant. The effect of temperature on ozone levels can vary depending on the initial pollutant concentrations. Hot and sunny weather also facilitates the formation of ground-level ozone, which is a significant contributor to poor air quality, particularly during heat waves in cities and nearby rural areas.

In contrast, cold temperatures during winter can exacerbate air quality issues. Low-pressure systems associated with cold weather bring windy and wet conditions, which can disperse pollutants over a wider area rather than effectively removing them. High-pressure systems, on the other hand, can lead to stagnant air, causing pollutants to concentrate in a specific area. The visibility of exhaust from vehicles, chimneys, and smokestacks is often higher during cold weather.

The interaction between weather and pollution is further complicated by other factors. For instance, humidity can mitigate the negative effects of ozone pollution. Thunderstorm clouds block sunlight, slowing down ozone production, while the moisture from storms helps destroy existing ozone. Drought conditions during heat waves increase the risk of forest fires, which release carbon monoxide and particulate matter, significantly degrading air quality.

Overall, the dynamic relationship between weather conditions and pollution formation underscores the importance of continuous air quality monitoring and modelling. By understanding these interactions, we can better manage and control air pollution, thereby mitigating its environmental and human health impacts.

Air quality alerts and warnings

An AQI value of 50 or below represents good air quality, while a value over 300 is hazardous. When the AQI reaches unhealthy levels, it is essential to take protective measures, especially for sensitive groups of people. For example, during the recent air quality alert in the Northeast due to smoke from wildfires in Canada, residents were advised to stay indoors, avoid outdoor exertion, and use air conditioning to reduce exposure to polluted air.

It is important to pay attention to air quality alerts and take appropriate action to safeguard your health. When a code red is issued, indicating severe pollution, it is recommended to stay indoors, minimize physical activity, and avoid cooking activities that create additional smoke. For those at risk, a code orange or yellow may also require similar precautions.

Additionally, masks can be an effective measure to protect against respiratory viruses, which are often a factor in air quality warnings. By following the guidelines provided with air quality alerts, individuals can minimize their exposure to harmful pollutants and maintain their respiratory health.

Air quality monitoring and estimation

The Air Quality Index (AQI) is a representation of air pollution concentration levels. It was developed by the EPA to provide a simple, uniform way to report daily air quality conditions to the general public. The AQI is calculated by converting measured pollutant concentrations to a uniform index based on the health effects associated with a pollutant. The health benchmarks used for calculating the AQI are pollutant-specific and are established by the EPA through the National Ambient Air Quality Standards. The Clean Air Act requires the EPA to review these standards every five years.

AQI values are dependent on temperatures, precipitation, wind, and cloud cover. Weather conditions can increase the rate at which air pollutants are formed or accumulate in the air. For example, ozone is formed by a chemical reaction between nitrogen oxides and volatile organic compounds in the air, and hot, sunny weather speeds up the process. Similarly, high humidity or pressure, strong overnight temperature inversions, or low wind speeds can cause fine particle pollution to increase. Fine particles are emitted from cars or other pollution sources but are also created in reactions between sulfur oxides, nitrogen oxides, and ammonia in the air.

There are eight major pollutants to be taken into account for AQI calculation, including particulate matter (PM 10 and PM 2.5), carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), ammonia (NH3), and lead (Pb). To calculate the AQI, data for a minimum of three pollutants must be present, one of which should be either PM10 or PM2.5. AQI ranging from 0-500 has different concentrations for each pollutant and corresponding health effects. For example, the range of 0-50 defines air quality as good, with minimal or no impact on health. Readings in the moderate range of 50 to 100 may affect unusually sensitive people. When the AQI value is near or exceeds 101, an air quality alert is issued, indicating that current or forecasted conditions can be harmful to those sensitive to air pollution.

In certain cases, air quality conditions in areas without air pollution monitors are estimated based on the nearest air monitor, including those in surrounding states. However, these estimates may not always reflect localized air quality events, such as fires.

Frequently asked questions