What are air quality scales?
Countries or regions define air quality indexes and categorize the raw data into a descriptive rating scale. These indexes make it easier to identify the level of pollution and if there’s any associated risk.
Different countries and regions use different scales to report air quality based on local pollution and health considerations. There are dozens of local indexes used across the globe. For example, some states in Australia use a number-based system while others use a category-based system. Canada, US, and Japan define separate air quality indexes, as does the European Environment Agency.
As the air pollution worsens, public health risks increase. It especially affects children, the older adult population, and other at-risk populations. During times of poor air quality, governmental agencies generally provide health recommendations related to indoor and outdoor activities.
How air quality indexes are calculated
Air Quality Index (AQI) is the way different governments choose to communicate air quality to the public. It's a means to convert the level of different pollutants into one index in a digestible manner.
Common differences between indexes include:
- Number and type of pollutants: Different AQIs are based on different individual pollutants.
- Some common pollutants that are tracked include:
- Particulate Matter, like PM2.5 and PM10
- Ozone (O3)
- Nitrogen Dioxide (NO2)
- Sulfur Dioxide (SO2)
- Carbon Monoxide (CO)
- Different countries and regions measure different pollutants for the index definition. For example:
- Some common pollutants that are tracked include:
- Averaging times: Many official sources provide reporting based on averaged readings for defined time frames. These time frames could range from 1–24 hours.
- Pollutant concentration thresholds: Different AQIs apply their own interpretations of danger to different pollutant concentration levels.
- Dominant pollutants: AQIs define the dominant pollutant based on risk of exposure, i.e., which pollutant is harming people’s health the most right now. As AQIs assign different interpretations of danger to individual pollutants, you can find differences in terms of the dominant pollutant.
How Air Quality Index (AQI) near you is selected
Air quality levels are calculated based on air quality stations measurements. We provide you with a map of all stations in your area to get a more comprehensive picture of the air quality information. However, the air quality between the stations may vary, and the AQI level at the closest station to you doesn't necessarily reflect the AQI level at your specific location. To avoid confusion, we show a map-based view to display the AQI level at given stations around you.
Due to space constraints, several Google products do present a single-station reading. In that case, the AQI value is selected according to the measurement in the station closest to your location.
- Pollutant concentrations may vary over short distances and cause air quality readings to vary sometimes drastically between your location and the location of a station.
- A short delay (12 hours) in the air quality data reporting may be experienced in some cases, which during events of rapidly changing air quality might be felt.
- Each monitoring station may not measure every pollutant. This difference can sometimes lead to discrepancies between reported AQI, which are station-specific and reflect only pollutants measured at that station, and actual air quality.
- Discrepancies to other data sources can also be caused by temporal averaging of an AQI, especially at the beginning and end of high pollution events.
What are the most common types of outdoor pollutants?
The Air Quality local Indexes are based on measurements of air pollutants. The most commonly measured pollutants outdoors are:
- Particulate Matter (PM): Small solid particles and liquid droplets found in the air. PM10 and PM2.5 refer to particles with a diameter smaller than 10 micrometers and 2.5 micrometers, respectively. It’s emitted from motor vehicles, wood heaters, and industry. Fires and dust storms can also produce high concentrations of particulate matter.
- Nitrogen Dioxide (NO2): A gas and a major component of central city air pollution. It mainly comes from vehicles, industry, power stations, and heating.
- Ozone (O3): A gas found both in the stratosphere, where it protects us from harmful ultraviolet radiation and the troposphere, i.e., close to the ground, where it's a harmful air pollutant. Ozone is produced by a chemical reaction between the sun light, organic gasses, and oxides of nitrogen released by:
- Power plants
- Other sources
- Sulfur Dioxide (SO2): A toxic gas with a pungent, irritating odor. It can come from electric industries that burn fossil fuels, petrol refineries, cement manufacturing, as well as volcano emissions.
- Carbon Monoxide (CO): A gas that's derived from motor vehicles or machinery that burn fossil fuels.
All these pollutants have health implications when they occur in high concentration. For more information, refer to the World Health Organization (WHO) website.
What do the smoke plumes mean?
Important: The map might show yesterday’s smoke while today’s smoke is still being analyzed. In some cases, the AQI might be good while there’s a smoke plume. This can be caused in cases where the smoke plume doesn’t reach the ground surface and doesn’t affect the measured air quality.
Additional information about smoke in the US is provided based on satellites’ data from NOAA, available in Google Search and Maps.
The data includes medium and high levels of smoke density. Smoke plumes will be shown on the air quality map if data is available.