General section

1. 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.

2. 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:
       • The European AQI reports on the 6 separate pollutants mentioned above.
       • The India AQI reports the pollutants above and on ammonia (NH3).

   • 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.

    

3. What are the most common types of outdoor pollutants and their sources?

   The Air Quality local Indexes are based on measurements of air pollutants. The most commonly measured outdoor pollutants are:

   • Particulate Matter (PM): Small solid particles and liquid droplets found in the air. PM10 and PM2.5 are particles with a diameter smaller than 10 micrometers and 2.5 micrometers. 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 in the stratosphere. It protects us from harmful ultraviolet radiation and the troposphere. Ozone is a harmful pollutant produced by a chemical reaction between the sun light, organic gasses, and nitrogen oxides released by:
     • Cars
     • 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, and volcano emissions.
   • Carbon Monoxide (CO): A gas from motor vehicles or machinery that burns fossil fuels.

   All these pollutants have health implications when they occur in high concentration. Learn more about these in the World Health Organization (WHO) website.

    

4. What influences air quality?

   The air quality is complex and influenced by several factors, such as:

   • Weather conditions (Wind speed and direction, relative humidity, etc.)
   • Solar irradiance
   • Wildfires and other types of fires
   • Dust storms and dust emissions from agriculture
   • Emissions from industry and private households
   • Traffic emissions
   • Other physical and chemical atmospheric processes

Station-based solution specific

1. How an 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.

   Important:

   • 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.

2. 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.

    

    

3. Air quality data sources

   Google Air Quality products that reflect monitoring stations data directly contain information from the sources below:

   Australia

   • Australian Capital Territory Government
   • Environment Protection Authority (EPA) Victoria
   • Environment Protection Authority (EPA) South Australia
   • New South Wales Government
   • Northern Territory Environment Protection Authority (EPA)
   • Queensland Government
   • Government of Western Australia

   Brazil

   • CETESB

   Chile

   • SINCA

   India

   • Central Pollution Control Board (CPCB)

   Israel

   • Svivaaqm

   Mexico

   • AirNow

   Singapore

   • National Environment Agency Singapore

   South Korea

   • AirKorea

   US

   • AirNow
   • PurpleAir

Model-based solution specific

1. How an Air Quality Index (AQI) near you is selected

   Google applies its air quality model in order to show the air quality at your location.

   Please note, if you're viewing air quality for a city, e.g., you’ve searched for “weather in London,” the resulting air quality reading might be for a location far away from you, like in the city center, and won't accurately reflect the situation around you, even if you're in the same city.

   To get AQI for your location, click on "choose area" on the location header and change to use precise location.

2. What is the data source of Google’s Air Quality model, and is it accurate at all times?

   Google’s air quality model is based on a multi-layered approach (fusion approach) that combines data from various input sources and weights the layers in a sophisticated way. The input layers are governmental reference monitoring stations, commercial sensor networks, global and regional dispersion models, fire smoke and dust models, satellite information, traffic data, and auxiliary information, such as land cover and meteorology.

   The Google model provides air quality indices based on the most common pollutant concentrations mentioned above with the addition of NO, NOx, and Non-Methane Hydrocarbons (NMHC) in some cases. The model is calculated on a 500m × 500m grid.

   Pollutant data from governmental or reference monitoring stations is the base layer and most trusted information in the model, where it’s available. The model performs quality assurance on measurements that are collected from monitors all over the world to remove any irregular values and ensure high quality of data. Where there's a significant delay between the time of measurement and its publication, a nowcasting algorithm calculates pollutant concentrations for the present hour.

   Model limitations

   While every layer of information used by the Google model has errors associated with it, our approach significantly reduces the total error as the model conducts cross validation between different sources. However, every model has errors. Some examples for that may be:

   • A short delay (1–2 hours) in the air quality data in some cases.
   • Very local events (e.g., BBQ, burning house) which aren't detected by the model.
   • In some cases, the model may show the smoke a few miles from your location. Note that users might experience delayed AQ data in Mexico and Canada during specific times due to fire smoke.

3. When I view the air quality index value on the nearby governmental air quality monitoring station, it’s different from what I see on Google. Why is that?

   There may be differences between the air quality index values on governmental monitoring stations and on Google because of the following reasons:

   • Not all governmental stations measure all pollutants.
   • Governmental monitoring station data frequently has reporting delays, so sudden changes in air quality may be missed.
   • Governmental monitoring stations only measure what happens at the station’s location.

   Google’s model takes into account multiple data sources (see table 2) and predicts the air quality at the location of the station at real time for all six pollutants:

   • Ground Level Ozone (O3)
   • PM2.5
   • PM10
   • Carbon Monoxide (CO)
   • Sulfur Dioxide (SO2)
   • Nitrogen Dioxide (NO2) at a 500m grid resolution.

   Example 1: During a dust event (PM10), a monitoring station will show good air quality since it only measures Ground Level Ozone (O3). However, the Google model shows poor air quality since PM10 is included in it.

   Example 2: A monitoring station measures all pollutants, Google and the station show the same dominant pollutant (for example, Ozone), however Google is showing an air quality index (AQI) of 200 while the station shows 150. This may be because Ozone changes throughout the day, so the measurement that was taken 2 hours ago may not be identical to Google’s real-time prediction.

4. The air quality that Google shows doesn't match with what I see on my (or nearest) commercial sensor. Why is that?

   The number of pollutants reported

   Most commercially available sensors report on PM2.5 and PM10 only, whereas Google provides air quality reporting for multiple pollutants, including Ground Level ozone (O3), PM2.5, PM10, Carbon Monoxide (CO), Sulfur Dioxide (SO2), and Nitrogen Dioxide (NO2). Note that in most commercial sensors, there's a decreasing efficiency for larger particles, meaning the PM10 measurement reliability may be very low due to the optical method limitation used in these sensors.

   Environmental factors

   External environmental factors, such as high relative humidity and temperature, may affect the reading of commercial optical sensors.

   The location of the sensor can also have a major effect on the readings as they can be impacted by very local pollution, which isn't representing a wider area.

   Google’s model incorporates readings from commercial sensors and identifies and removes invalid measurements.

   The conversion process involved

   Commercial sensors often adopt a “count-based” method of PM2.5 measurement, while the standard format used for reporting air quality information is “mass-based.”

   The government monitoring stations and Google’s model report in terms of “Mass per Volume” by default. This conversion requires the density of the particle (meaning which type of particle, smoke, dust, etc.). This conversion may lead to large differences when compared to government monitors and Google’s model.

   The averaging time

   Different commercial sensor network providers display data with different averaging times. Google is calculating pollutant concentrations and AQIs on an hourly basis. Additionally, each AQI itself has their own averaging times (see below based on the country), which is usually at least hourly, often multiple hours. For example, in case of a sudden increase in smoke pollution, you may see poor air quality being indicated on a commercial sensor provider website based on a 10-minute average, while Google would show the hourly average (which in this case would be lower because of the preceding hours of lower pollution) and the official AQI value which reflects health implications and is based on even longer averages (see below).

   Tip: The commercial sensor network isn't included in our model and may show different results.

5. Why does Google’s Air Quality information look different when I compare other air quality providers?

   Different air quality indexes

   Different countries and regions use air quality indexes based on different scales, for different purposes. If you’re looking to compare providers, it’s important to ensure you're comparing reports that speak the same “air quality language.”

   For example, some providers use the US AQI hourly, while others use the US AQI as set (e.g., averaging PM2.5 over 24 hours) or the AirNow index.

   Different reporting and measurement methods

   There are differences in how different providers measure air quality, which affects the final results.

   In some cases, air quality data is based on measurements. Some providers use commercial sensors, and others use models.

   Google combines information from monitoring stations around the world with commercial sensor information, satellite data, weather patterns, traffic conditions reporting, wildfire and land cover information.

   Different providers might have different time aggregations when reporting air quality data, so it’s possible to see time delays in reported pollutant or index values across different providers at the same location.

6. You’re showing good air quality, but I see or smell smoke or pollution in front of me, why?

   Sometimes, the air quality information you see on Google might not seem consistent with what you see or smell around you. In our experience, there are usually a number of possible explanations:

   • Our noses are particularly sensitive to smell. It’s possible that you can smell a particular type of pollution, like gasses from smoke in very low concentrations, even if the air quality is considered safe enough from a health perspective. Air pollution created by Volatile Organic Compounds (VOCs) can produce noticeable smells, but aren't measured by government monitoring stations or included in our air quality reports.
   • Smoke that affects visibility can often be seen at high altitudes even if it's not detectable at the ground level.
   • Google model limitation, as described above.

7. You’re showing bad air quality, but I see that everything is clear and sunny, why?

   While some types of pollution are visible to us, such as dust storms or wildfire smoke, a lot of air pollution isn't visible to the naked eye. This is one of the fundamental ways air pollution differs from the weather, where often we can simply glance outside and see if it's sunny, rainy, windy, etc.

   For example, high levels of ozone (otherwise known as the “beautiful day” effect), which requires the presence of sunlight to form, can be created at high altitudes (for example, on a mountain top) on an otherwise nice and sunny day.

   Another reason could be the model limitation as described above.

8. Why does Google report differently to AirNow in the US?

   The AirNow main website and the AirNow Fire and Smoke Map are 2 of the most popular official sources for air quality information in the United States.

   
   There are some key differences between AirNow and Google air quality reports:

   • Number of pollutants monitored: Google is aligned with the US EPA (using US EPA data) and covers more pollutants than AirNow shows in the website (see table below).
   • Google uses more data sources (see table).
   • Google reports hyper-local information versus area-wide worst report by AirNow.
   • Google and Air Now have differences in the Air Quality Index (AQI) (see below).

     	AirNow	AirNow Fire and Smoke Map	Google
     Data sources and method	Monitoring stations and interpolation model based on the worst reading in a region	Monitoring stations and commercial sensors Modeling is applied to report on wildfire smoke	Various input data, models and different temporal and spatial prediction algorithms: Monitoring stations Commercial sensor Satellite data Weather patterns Traffic conditions Wildfire tracking Land cover information
     Pollutants monitored	Ground Level ozone (O3) (NowCast) Particulate Matter (PM2.5)(NowCast) Particulate Matter (PM10)(NowCast) Carbon Monoxide (CO) Sulfur Dioxide (SO2) Nitrogen Dioxide (NO2)	Particulate Matter (PM2.5)	Ground Level Ozone (O3) Particulate Matter (PM2.5) Particulate Matter (PM10) Carbon Monoxide (CO) Sulfur Dioxide (SO2) Nitrogen Dioxide (NO2)

   The US has 2 official ways of calculating the Air Quality Index:

   • The US EPA’s AQI, which calculates the exposure over longer periods of time and incorporates six  pollutants (see table below).
   • NowCast, which is used to approximate the complete daily AQI during any given hour. It provides current conditions to enable people the power to take action to reduce outdoor activities and exposure when necessary and protect their health. The NowCast allows current conditions maps to align more closely with what people are actually seeing or experiencing. It's used to provide more timely information for Ozone, PM2.5, and PM10.

   When reporting the AQI in the US, Google combines the 2 indexes.

   Comparing Official USA AQIs & Google

   Here's a more detailed breakdown of the different US AQI approaches, so you can compare them side by side:

   Key Differences

   	US AQI	AirNow NowCast	Google Hybrid US AQI
   Number of pollutants	6 Pollutants Ground Level Ozone (O3) Particulate Matter 2.5um (PM2.5) Particulate Matter 10um (PM10) Carbon Monoxide (CO) Sulfur Dioxide (SO2) Nitrogen Dioxide (NO2)	3 Pollutants Ground Level Ozone (O3) Particulate Matter 2.5um (PM2.5) Particulate Matter 10um (PM10)	6 Pollutants Ground Level Ozone (O3) Particulate Matter 2.5um (PM2.5) Particulate Matter 10um (PM10) Carbon Monoxide (CO) Sulfur Dioxide (SO2) Nitrogen Dioxide (NO2)
   Time periods considered	The average time period covered varies by pollutant: e.g., ozone is calculated based on 8-hour average exposure levels as well as 1-hour exposure ranges. PM2.5 is reported based on 24-hour exposure averages.	The AirNow NowCast averaging method gives more weight to recent hours to better reflect sudden changes in air pollution levels due to smoke from fires or dust storm events. Shorter average time periods compared to US AQI e.g., taking into account the last period of 12 hours.	Google adopts the AirNow NowCast averaging method for Particulate Matter (PM10 and PM2.5), giving more weight to recent hours and sudden changes. But we also report on more pollutants compared to NowCast: Google takes the averages of other pollutants: O3, NO2, SO2, CO, runs calculations, then converts this into the US AQI format.

9. Air quality data sources

   Google Air Quality model contains information available from the sources below:

   Global data sources

   • Low-cost sensors data from PurpleAir.
   • Modified Copernicus Atmosphere Monitoring Service information.
   • Modified Copernicus Global Land Cover information.
   • Met Office. Contains public sector information licensed under the Open Government Licence v3.0.
   • European EEA information under CC-BY-2.5 DK license.

   Belgium

   • Modified IRCEL - CELINE information. License. 

   Canada

   • Information from the Manitoba government, licensed under the OpenMB Information and Data Use Licence (Manitoba.ca/OpenMB).
   • Information licensed under the Open Government Licence – Ontario, version 1.0.

   Denmark

   • DCE - National Center for Miljø og Energi. The data is a raw data that isn't quality controlled. 

   Finland

   • ilmatieteen laitos.

   France

   • Pays de la Loire: Source of the data: Air Pays de la Loire.
   • Geo D'Air.

   Germany

   • Contains modifiedGerman Environment Agency information. Changes were made.

   Guernsey

   • © Crown 2023 copyright Defra via uk-air.defra.gov.uk.

   Italy

   • Regione del Veneto. License.

   Ireland

   • Environmental Protection Agency (EPA): https://epa.ie/ https://airquality.ie/.
   • License. 

   Japan

   • Modified Soramame information.

   Mexico

   • The Air Quality information published by the Environment Secretariat of the Government of Mexico City is prepared from the information obtained from the Atmospheric Monitoring Network and its monitoring stations in the Metropolitan Zone of the Valley of Mexico, which are operated and managed by the Air Quality Monitoring Directorate of the General Directorate of Air Quality (SEDEMA). This information is public and free, and it is subject to quality processes that could modify it. The dissemination or use of this information by third parties is under the responsibility of the person who publishes or uses it.
   • SINAICA, https://sinaica.inecc.gob.mx/. Changes were made.

   Spain

   • MeteoGalicia and the Ministry of the Environment, Territory and Housing of the Xunta de Galicia.
   • Madrid Comunidad.
   • Ministry of the Environment information under CC BY 4.0 license.

   Sweden

   • Contains modified SMHI information.

   UK

   • © Crown 2023 copyright Defra via uk-air.defra.gov.uk.
   • Contains LondonAir information. License.
   • Northern Ireland Air.

   US

   • Texas TCEQ.
   • New York State, Department of Environmental Conservation: The data displayed here includes data obtained from http://nyaqinow.net that's preliminary and subject to change.