AirNow’s fire map includes data from PurpleAir sensors (that’s what the little circles represent), and Watch Duty, a nonprofit fire tracking app, also displays PurpleAir data. But probably due to different calculations and processing delays, the air quality index reported by the same sensors can vary greatly from map to map. For what’s known as PM2.5 pollution, or tiny respirable particles from smoke and dust, a PurpleAir sensor south of Los Angeles International Airport simultaneously has an air quality index of 28 on AirNow’s website, 20 on WatchDuty and returned 5 on PurpleAir’s. official page.

Each of those values ​​generally indicates healthy air, but things can get more complicated when other types of data are added to the calculations. That’s exactly what companies like BreezoMeter and Ambee are doing, hoping to provide what they describe as accurate “hyperlocal” estimates in the many kilometers between some sensor locations.

BreezoMeter was founded in Israel and raised tens of millions of dollars in venture capital funding before being acquired by Google in 2022 for more than $200 million, according to Israeli media. (Google declined to comment on the deal value.) It powers the air quality data seen in the Weather app on Apple devices and the Google Maps app. Indian startup Ambee, meanwhile, is responsible for air quality data in the app WeatherBug, which is one of the most popular weather apps in the world.

Yael Maguire, vice president of geo-sustainability at Google, says BreezoMeter estimates air quality on an hourly basis across a wide range of pollutants and locations, generating more data than many government systems. To make its calculations, the company uses information obtained not only from the EPA and PurpleAir sensors, but also satellites and other sources such as weather and traffic reports. Similar data is included in Ambee’s own algorithm, according to its CEO Jaideep Singh Bachher. “We want to give people the right data whenever and wherever they need it,” he says.

Volckens says he doesn’t trust these systems. The low-cost PurpleAir sensors they use in part are not well-suited to the conditions where they are often deployed in the US, including during wildfires, he says. But he admits that while they tend to be off in terms of the raw numbers, these sensors can be accurate about 90 percent of the time in determining the advisory level — that green-to-maroon scale that’s often enough for people to make decisions about how to protect their health.

Andrew White, a representative for PurpleAir, says its sensors have been found to be precise and that it is not in control of how other services perform calculations using its data. Google’s Maguire says the company “provides industry-leading, highly accurate air quality information,” including “even in areas with limited monitoring.”

Jennifer Richmond-Bryant, an associate professor of forestry and environmental resources at North Carolina State University who has studied PurpleAir sensors, says the safest bet for anyone concerned about air quality is to trust the highest number or color level among different services. “I have more confidence in the AirNow numbers because I understand them,” she says. But “it can never hurt to make an overly conservative decision when it comes to protecting yourself.”