Kiera Jefferson
The Air Quality Index (AQI) is a unitless number that simplifies the complex science of pollution composition, exposure rate-based health severity, ambient standards, and measurement protocols into simple color-coded alerts for air pollution categories. Various tools and calculators are available online to estimate the AQI, such as the AQI Calculator provided by AirNow.gov. The AQI value can be calculated per hour or per 24 hours, and it is based on pollutant concentration data. Different countries and regions may have their own specific methodologies and standards for calculating air quality indices, and it is important to refer to the relevant guidelines and resources for accurate calculations.
| Characteristics | Values |
|---|---|
| Name of Pollution Index | Air Quality Index (AQI) |
| What it measures | How clean or polluted the air is |
| Components | Six atmospheric pollutants: sulfur dioxide (SO2), nitrogen dioxide (NO2), suspended particulates smaller than 10 μm in a, PM10, PM2.5, ozone |
| Scale | 1 to 10+ |
| Categories | Good, Satisfactory, Moderate, Poor, Severe, Hazardous |
| Calculation | Refer to the AQI Technical Assistance Document for equations |
| Tools | MS-Excel-based calculator |
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What You'll Learn
Calculating the Air Quality Index (AQI)
The Air Quality Index (AQI) is a unitless number that simplifies the complex science of pollution composition, exposure rate-based health severity, ambient standards, and measurement protocols into simple colour-coded alerts for the general public regarding the severity of air pollution. According to the Indian Government (CPCB), the Indian AQI ranges from 0 to 500, with 0 being good and 500 being severe.
To calculate the AQI, data for a minimum of three pollutants must be present, with one being either PM10 or PM2.5. These pollutants include particulate matter (PM10 and PM2.5), carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), ammonia (NH3), and lead (Pb).
The Indian Government and the US-EPA both use PM10 and PM2.5 as criteria for AQI calculation. The safe exposure levels for PM10 (24 hours) are 0-100 ug/m3 and for PM 2.5 (24 hours) is 0-60 ug/m3 as per the Indian CPCB. As per US-EPA, safe levels of PM10 are 0-54 ug/m3 and PM 2.5 is 0-12.0 ug/m3. Sulfur dioxide is another parameter used by both entities, with safe exposure levels of 0-80 ug/m3 (24 hours) and 0-75 ppb (1 hour) according to the Indian government and US-EPA, respectively.
To calculate the AQI for other parameters, you can apply the equation differently and identify the worst-hit index to determine the overall AQI.
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Understanding the Pollution Load Index (PLI)
The Pollution Load Index (PLI) is a tool used to evaluate the extent of heavy metal (HM) pollution in soils and sediments. It was first presented by Tomlinson et al. in 1980 and has since been used by many researchers to identify contamination levels in environmental matrix analyses.
PLI is calculated as the nth root of the number of multiplied CF (contamination factor) values. The formula for calculating the CF values is given by: CF = (M/N)sample/(M/N)baseline, where M is the concentration of the metal and N is the normalizer, for both the sample and nearby non-polluted samples of similar texture. The CF values indicate the level of contamination, with CF < 1 indicating low contamination, 1 ≤ CF < 3 indicating moderate contamination, 3 ≤ CF ≤ 6 indicating considerable contamination, and CF > 6 indicating severe or very high contamination.
The PLI values range from 0 to 1, with values less than 1 implying no contamination and values greater than 1 indicating severe heavy metal pollution. For example, in the AWS environment, the site-specific PLI values were estimated to be between 0.24 and 1.53, with 47% of samples having PLI values less than 1 and 53% of samples having PLI values greater than 1.
While the PLI is a useful tool for assessing the overall degree of contamination, it should be noted that it softens the differences between the evaluated metals and can lead to a misinterpretation of the real state of the sediments under study. Therefore, it should be used as a complementary tool alongside other indices and expert evaluations of individual parameters to improve the understanding of the metals being analysed.
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Calculating the Index of Geo-accumulation (Igeo)
The Index of Geo-accumulation (Igeo) is a quantitative index used to identify the accumulation and pollution status of heavy metals in soils, considering background values. It is calculated using the formula: Igeo = log2(Cn/1.5Bn). Here, Cn represents the concentration of metal in the study area, and Bn represents the background value of the same metal.
To calculate the Igeo in Excel, you will need to set up your spreadsheet with three columns (A, B, and C). In column A, input the sediment concentration values. In column B, input the concentration of the reference sample. To calculate the values for column C (Igeo), use the formula: C = log((A/1.5)*5), assuming 5 is the reference.
For example, if the value in cell A1 is 10 (representing the sediment concentration), and the value in cell B1 is 6 (representing the concentration of the reference sample), the formula in cell C1 would be: C1 = log((10/1.5)*5). The resulting value in cell C1 would be approximately 1.76271.
The Igeo value provides an indication of the level of pollution. According to Ren et al. (2022), the Igeo values can be classified into five categories: Igeo ≤ 0 (unpolluted), 0 < Igeo ≤ 1 (slightly polluted), 1 < Igeo ≤ 2 (moderately polluted), 2 < Igeo ≤ 3 (heavily polluted), and Igeo > 3 (severely polluted).
It is important to note that the calculation of Igeo assumes that the background concentration of chemicals created entirely by humans, such as chlorinated biphenyls or organochlorine pesticides, is zero. For chemicals with both anthropogenic and natural components, such as metals, the human component must be determined and subtracted from the total concentration to obtain the natural background concentration.
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Using breakpoint concentrations to calculate the Individual Air Quality Index (IAQI)
The Individual Air Quality Index (IAQI) is a useful metric for understanding air quality in various settings, from schools to offices and homes. Unlike the Air Quality Index (AQI), which is used to report outdoor air quality, the IAQI amalgamates data from various sources to provide a comprehensive snapshot of overall indoor air quality.
The IAQI is calculated using a breakpoint table and a linear interpolation formula to convert from concentration to AQI. The breakpoint table defines the low and high concentration breakpoints for each pollutant, and the corresponding low and high index values. The formula for calculating the index for a pollutant is given by:
IP = (Ihigh - Ilow) / (Chigh - Clow) * (CP - Clow) + Ilow
Where:
- IP is the index for the pollutant
- Ihigh and Ilow are the high and low index values
- Chigh and Clow are the high and low concentration breakpoints
- CP is the concentration reading for the pollutant
To determine the real-time IAQI value, the individual index for each pollutant is calculated using the above formula. Each pollutant is assigned a status based on its individual index value, such as "Good", "Moderate", or "Polluted". The final IAQI value is then determined by comparing the individual indices and selecting the lowest index value.
It is important to note that the IAQI scale is different from the U.S. Environmental Protection Agency's (EPA) AQI scale. On the IAQI scale, a higher value indicates better air quality, whereas on the EPA's AQI scale, a higher value represents worse air quality. Additionally, the IAQI uses a one-minute average for index calculation, providing more immediate readings compared to the EPA's one- or eight-hour pollutant averages.
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Calculating the National Air Quality Index (NAQI)
The National Air Quality Index (NAQI) is used to communicate to the public how polluted the air currently is or how polluted it is forecast to become. As air pollution levels rise, so does the NAQI, along with the associated public health risk. There are six NAQI categories: Good, Satisfactory, Moderate, Poor, Severe, and Hazardous.
NAQI considers eight pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb. For each of these pollutants, a sub-index is calculated based on measured ambient concentrations, corresponding standards, and likely health impacts. The worst sub-index reflects the overall NAQI. The likely health impacts for different NAQI categories and pollutants are suggested by medical experts.
To calculate the NAQI in Excel, you would need to obtain data for the ambient concentrations of the eight pollutants. This data can be collected from air quality monitors or models. Once you have the concentration data, you can calculate the sub-index for each pollutant using the following formula:
= POLLUTANT_CONCENTRATION * POLLUTANT_STANDARD * HEALTH_IMPACT_FACTOR
Where:
- POLLUTANT_CONCENTRATION is the measured ambient concentration of the pollutant.
- POLLUTANT_STANDARD is the corresponding standard for that pollutant.
- HEALTH_IMPACT_FACTOR represents the likely health impact of that pollutant.
After calculating the sub-index for each pollutant, you can determine the overall NAQI by taking the worst sub-index value. It is important to note that the specific formula and weighting factors may vary depending on the country and the air quality standards being used.
Additionally, when calculating the NAQI, it is essential to consider the time dimension. The concentration of air pollutants may vary throughout the day or over different periods. Therefore, the averaging period for the concentration data should be specified, typically ranging from hourly to annual averages. The time dimension is crucial in understanding the dose of air pollutant exposure and its potential health effects.
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Frequently asked questions
AQI stands for Air Quality Index. It is a unitless number that simplifies the science of pollution composition, exposure rate-based health severity, ambient standards, and measurement and standard protocols into simple color-coded alerts of good, moderate, unhealthy, etc. air pollution categories.
First, select a pollutant, then enter the AQI value. Each pollutant is given a sub-index, and the worst sub-index reflects the overall NAQI. The final AQI value can be calculated per hour or per 24 hours and is the maximum of these six scores. The AQI Technical Assistance Document provides equations for calculating the AQI from pollutant concentrations.
Some examples of pollutants with associated AQIs include ground-level ozone, particulates, sulfur dioxide, carbon monoxide, and nitrogen dioxide.
Other pollution indices include the Pollution Load Index (PLI), the Index of Geo-accumulation (Igeo), and the Contamination Factor (CF). Each index has its own formula for calculating it, and some require geochemical background values and quality standards.
