AirNow's fire map includes data from PurpleAir sensors (that's what the little circles represent), and Watch Duty (a nonprofit app for tracking fires) also displays PurpleAir data. But the air quality index reported by the same sensor can vary significantly on different maps, possibly due to different calculation and processing delays. For so-called PM2.5 pollution, which are tiny, respirable smoke and dust particles, a PurpleAir sensor located just south of Los Angeles International Airport simultaneously returned an air quality index of 28 on the AirNow website on Monday, 20 on the WatchDuty website, and 20 on the PurpleAir website The above is the official page of 5.

Each of these values ​​generally indicates healthy air, but when other types of data are added to the calculation, things can get more complicated. That's exactly what companies like BreezoMeter and Ambee are doing, hoping to provide what they describe as accurate "hyperlocal" estimates over many miles between a few sensor locations.

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

Yael Maguire, vice president of sustainability at Google Geo, said BreezoMeter estimates air quality hourly for a variety of pollutants and locations, generating more data than many government systems. To make its calculations, the company uses information gathered not only from the EPA and PurpleAir sensors, but also from satellites and other sources such as weather and traffic reports. Ambee CEO Jaideep Singh Bachher said similar data is incorporated into its proprietary algorithm. “We want to get the right data to people anytime, anywhere,” he said.

Walkens said he doesn't trust the systems. He said the low-cost PurpleAir sensors they used in part were not well suited to the conditions in which they are often deployed in the United States, including during wildfires. But he acknowledged that while they tend to be biased in their raw data, the sensors are accurate about 90 percent of the time in determining advisory levels — a green-to-maroon scale that is often enough to make decisions about how to protect their health.

Andrew White, a representative for PurpleAir, said its sensors are very precise and it has no control over how other services use its data for calculations. Google's Maguire said 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 studies PurpleAir sensors, said 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 AirNow's data because I know them," she said. But "it never hurts to make an overly conservative decision when it comes to protecting yourself."