Understanding Air Quality: Calculating The Aqi

The Air Quality Index (AQI) is a tool developed by the US Environmental Protection Agency (EPA) to convey how clean or polluted the air is. It is calculated by converting measured pollutant concentrations to a uniform index based on the health effects associated with a pollutant. The Indian Government's AQI range is from 0-500, with 0 being good and 500 being severe. The US-EPA's AQI includes six color-coded categories, each corresponding to a range of index values, with an AQI value of 50 or below representing good air quality and a value over 300 representing hazardous air quality. The pollutants considered for the calculation of AQI include particulate matter (PM 10 and PM 2.5), carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), ammonia (NH3), and lead (Pb).

AQI calculation parameters

The Air Quality Index (AQI) is a tool developed by the U.S. Environmental Protection Agency (EPA) to communicate information about outdoor air quality and health to the public. The AQI is calculated by converting measured pollutant concentrations to a uniform index based on the health effects associated with a particular 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 is calculated using separate equations for parameters. For example, if you wish to calculate the AQI based on four parameters, use the equation four times, and the worst sub-index is the AQI. A sub-index is a linear function of the concentration of pollutants. The formula for calculating the AQI is:

Ip = [IHi – ILo / BPHi – BPLo] (Cp – BPLo) + ILo

Where:

• Ip = index of pollutant p
• Cp = truncated concentration of pollutant p
• BPHi = concentration breakpoint (greater than or equal to Cp)
• BPLo = concentration breakpoint (less than or equal to Cp)
• IHi = AQI value corresponding to BPHi
• ILo = AQI value corresponding to BPLo

The Indian Government (CPCB) and the US-EPA both consider particulate matter (PM10 and PM2.5) as criteria for AQI calculation. Particulate matter is released from construction, smoking, cleaning, renovations, demolitions, natural hazards, and industrial emissions. These particles can cause respiratory ailments such as asthma, coughing, and irritation of the airways, eyes, and nose when inhaled.

Other pollutants considered by the Indian Government and the US-EPA for AQI calculation include carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and lead (Pb). The Indian Government also includes ammonia (NH3) and lead (Pb) in their AQI calculations.

Pollutant concentration conversion

The Air Quality Index (AQI) is a scale developed by the US Environmental Protection Agency (EPA) to convey how clean or polluted the air is. The AQI is usually reported with a colour code that indicates the air quality with respect to the pollutant being reported. Green shows good air quality, yellow shows moderate air quality, orange shows unhealthy for sensitive groups, red shows unhealthy, purple shows very unhealthy, and maroon shows hazardous.

To calculate the AQI, you first need to measure the concentration of pollutants in the air. This can be done using instruments such as the BAM 1020 Beta Attenuation Mass Monitor, which is designated by the US EPA for continuous PM2.5 monitoring. PM2.5 refers to particulate matter that is less than 2.5 microns in diameter, which is small enough that it cannot be seen by the human eye. However, when present in large concentrations, PM2.5 can reduce visibility and pose a serious health risk.

The BAM 1020 monitor can measure PM2.5 levels with a 1-hour time resolution continuously. This data can then be used directly for air quality assessment or converted into the AQI or NowCast index values for public reporting purposes. The NowCast AQI is generally a better predictive tool for anticipating future values as it can account for massive changes in the reported AQI from one day to the next.

It's important to note that there is currently no US-EPA hourly standard for PM2.5, but the daily concentration standard for PM2.5 is 35 μg/m3.

Health benchmarks

The Air Quality Index (AQI) is a tool developed by the EPA to communicate information about outdoor air quality and health. The AQI is calculated by converting measured pollutant concentrations to a uniform index based on the health effects associated with a particular pollutant. The health benchmarks used for calculating the AQI are pollutant-specific and 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 provides a simple and uniform way to report daily air quality conditions. It runs from 0 to 500, with higher values indicating greater air pollution and more significant health concerns. An AQI value of 50 or below represents good air quality, while an AQI value over 300 indicates hazardous air quality. Values above 100 indicate unhealthy air quality, initially for sensitive groups of people and then for everyone as values increase.

The AQI includes six color-coded categories, each representing a range of index values and corresponding to a different level of health concern. The colors enable people to quickly determine whether the air quality in their community is reaching unhealthy levels. An air quality alert is issued when measured or forecasted air quality conditions exceed 101 on the index, indicating that current or predicted conditions may be harmful to individuals sensitive to air pollution.

It is important to note that most poor air quality days are not caused by short-term increases in emissions from industries, cars, or other sources. Instead, air quality alerts are often driven by weather conditions that increase the rate of air pollutant formation or accumulation. For example, ozone, a major air pollutant, is formed by a chemical reaction between nitrogen oxides and volatile organic compounds in the air, and this process is accelerated by hot and sunny weather. 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 and other pollution sources but can also be created by reactions between sulfur oxides, nitrogen oxides, and ammonia in the air.

Air quality categories

The Air Quality Index (AQI) is a yardstick that runs from 0 to 500, with higher values indicating increased air pollution and health risks. The AQI was developed by the EPA and is used to convey how clean or polluted the air is in a given location. The index is colour-coded, making it easy for people to quickly assess the air quality in their communities.

There are six levels of health concern within the AQI:

• "Good" AQI is 0-50: Air quality is considered satisfactory, and air pollution poses little to no risk.
• "Moderate" AQI is 51-100: Air quality is acceptable, but certain pollutants may pose a moderate health risk to a small number of people. For example, those unusually sensitive to ozone may experience respiratory symptoms.
• "Unhealthy for Sensitive Groups" AQI is 101-150: While the general public is unlikely to be affected, people with lung disease, older adults, and children are at a higher risk from exposure to ozone and particles in the air.
• "Unhealthy" AQI: Air quality is considered unhealthy for everyone as AQI values exceed 150.
• "Very Unhealthy": This category indicates that air pollution has reached levels that are unhealthy for everyone and may cause significant health issues for sensitive groups.
• "Hazardous" AQI is over 300: This category represents dangerous air quality levels that pose serious health risks to the general public.

The AirData website provides access to monitored air quality data across the United States, Puerto Rico, and the U.S. Virgin Islands. It offers various tools for visualizing data, such as the AQI Plot, Tile Plot, and Concentration Plot, which help users understand air quality trends and the number of unhealthy days throughout the year.

AQI ranges

The Air Quality Index (AQI) is a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the more serious the health concern.

An AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality. There are six separate AQI levels:

• Good
• Moderate
• Unhealthy for Sensitive Groups (USG) (101-150 AQI): At this level, the general public is unlikely to be affected. However, people with lung disease, older adults, and children are at greater risk from exposure to ozone. Those with heart and lung disease, older adults, and children are also more vulnerable to the presence of particles in the air.
• Unhealthy (151-200 AQI): At this level, everyone may begin to experience adverse health effects, and members of sensitive groups may experience more serious consequences.
• Very Unhealthy (201-300 AQI): A health alert is triggered, signifying that everyone may experience more serious health effects.
• Hazardous (over 300 AQI): Emergency health warnings are triggered, indicating that the entire population is likely to be affected.

The U.S. EPA establishes an AQI for five major air pollutants regulated by the Clean Air Act. Each pollutant has a national air quality standard set by the EPA to protect public health.

Frequently asked questions