AirNow vs. PurpleAir Air Quality Monitoring for Constituents of Wildfire Smoke – How do Data Sources Differ and Why does it Matter?

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The demand for timely, representative, and accurate air quality monitoring has increased in recent years, in large part, due to the rise in the intensity and frequency of wildfires in areas such as the western United States. To date, demand has primarily been driven by regulatory and consumer forces. For example, as described in our previous blog, Wildfire Smoke as an Occupational Health Hazard: Regulatory Update and Current State of the Science, the California Occupational Safety and Health Administration (Cal/OSHA) issued an emergency regulation to protect workers from exposure to wildfire smoke effective as of July 28, 2019, that requires employers to determine employee exposure to wildfire smoke using either the Air Quality Index (AQI) or their own direct reading instruments. In addition, the availability of crowd-sourced air monitoring data, such as real-time data reported from PurpleAir’s air quality monitoring network, has increased in connection with consumer demand.

Real-time reporting and the increasing availability of data have allowed employers and the public greater access to air quality data through multiple sources, including monitoring websites such as PurpleAir and AirNow. However, these websites may present different air quality information at times, potentially causing confusion as to the “true” air quality at a given location and time. Discussing the differences between PurpleAir and AirNow is essential for users to understand the current air quality conditions and take precautionary measures, if necessary. See the table below for a summary of these two monitoring systems.

Differences between AirNow and PurpleAir




Averaging Times

Offers real-time, 10, 30, 1-hour, and 1-week averages

Utilizes the NowCast algorithm, which updates hourly air quality readings

Pollutants Monitored







Some locations also provide real-time forecasts on NO2 and CO

Measuring Techniques

Sensors measure aerosol PM in real-time using laser counters

Measures PM mass concentrations by processing air through a filter and then weighing the filter to determine air pollutant concentrations

Spatial Distribution

Sensors are low-cost, can be purchased by anyone, placed indoors or outdoors with the data collected displayed on a virtual map on the PurpleAir Website

Monitors are located at fixed sites and are typically distributed in regions with larger populations

Conversion Factors

-Studies have found that PurpleAir monitors tend to yield higher values than the AirNow AQI; observed discrepancies have been attributed to differences in PurpleAir vs. AirNow measurement techniques.

-Multiple correction factors (e.g. AQ and U, Lane Regional Air Protection Agency (LRAPA), and the US EPA) were developed as a means to adjust PurpleAir data for better agreement with AirNow data; these factors are based on side-by-side studies of AirNow and PurpleAir methodologies.

NowCast algorithm

PM = particulate matter (; AirNow NowCast; PurpleAir Website FAQ; AirNow Website Map; AirNow Data; PurpleAir Website FAQ; Moffitt, 2020; PurpleAir FAQ; Dryer, 2020; AirNow NowCast; Hug, 2020; PurpleAir Map; EPA Correction Factor, 2020; Study of PurpleAir Sensors; AQ and U; Sayahi, 2019;Kelly 2017)

When considering air pollutant exposures, such as airborne exposure to wildfire smoke during occupational and recreational activities, it is important for employers and the public to have access to accurate and timely air quality information. As such, knowing and understanding the differences between PurpleAir and AirNow, and utilizing appropriate conversion factors are critical in getting accurate readings for air quality and making well-informed risk management decisions.

Cardno ChemRisk scientists have evaluated exposure profiles and dose-response relationships for complex mixtures resulting from wildfires, air pollution, and industrial emissions. Cardno ChemRisk has a number of industrial hygienists and environmental health professionals who can assess exposure and risk of adverse health effects in specific settings. If you have any questions or would like more information about our environment, health, and safety capabilities, please contact Rachel Zisook.