Air quality in the United States and Europe improved over the last decade thanks to stricter environmental regulations, but intensifying wildfires raise new air pollution concerns.

The National Interagency Fire Center reported that, as of August 8, there were 107 large active fires that had burned more than 2.2 million acres domestically across 15 states so far this year. In Europe, wildfires in Greece and Turkey are raging amid record heat waves now as well.

As a result, more people are turning to mobile apps to understand when air quality is better or worse, wherever they may be. These apps use a mix of data from government-operated satellites, or weather, fire and ambient air quality stations, as well as sensors and systems run by private sector entities. Some are even crowdsourced from relatively affordable air quality sensors sold by companies such as PurpleAir and IQAir.

According to senior mobile insights analyst Jonathan Briskman of Sensor Tower, the top-rated apps for outdoor air quality monitoring in the U.S. between January 2020 and July 2021 have been: AirCare, AirVisual, and South Coast AQMD, based on ratings from the App Store, and Google Play.

Here’s what those three apps do:

• AirCare, made by developers in Northern Macedonia, is available for iOS and Android mobile devices, including iPhones, iPads, Apple Watch and Huawei smartphones, among many others. Tiers include a free, ad-supported version, a 99 cents ad-free version, and at the premium level, a $14.99 annual subscription for a pro-version. The app includes kid-friendly air pollution information, charts and maps that show pollutant levels derived from government-run sensors and stations, alongside volunteers’ PurpleAir and other sensors throughout the U.S., Europe and Australia. In some major metro areas, the app also tracks ultra-violet and pollen levels.

• AirVisual, made by the Swiss air quality company IQAir, tracks air pollution in more than 10,000 cities and 80 countries drawing on data from tens of thousands of sensors, some positioned at U.S. embassies overseas. The company’s free mobile apps are also ad-free and available for iOS and Android devices. Besides real-time maps that show levels of six different types of major pollutants, IQAir’s AirVisual and mobile website provide seven-day air pollution and weather forecasts, along with air pollution-related news and health information. The apps can pair with the company’s own sensors, including the portable AirVisual Pro sold for around $269.

• South Coast AQMD, is a free and ad-free app run by the local air pollution agency in Southern California of the same name and tracks air pollution across Orange County, Los Angeles, Riverside and San Bernardino, specifically. It features real-time and forecasted air quality and weather conditions, and maps that show where drivers can charge their electric vehicles or find other non-traditional fueling stations. It also includes information about upcoming local events and political hearings related to air quality issues to encourage community participation. The app is available in both English and Spanish for Apple and Android devices.

The five most popular air quality apps in the U.S., based on installs since the start of 2020, according to Sensor Tower included two of those top-rated apps, AirVisual (from IQ Air) and Air Care, as well as the U.S. Environmental Protection Agency’s app AirNow, an app from venture-backed startup Breezometer that shows air quality, pollen and active fire data, and an app called Oregon Air developed for the Oregon Department of Environmental Quality.

Use of these apps, and new installs are often driven by regional events. As of August 8, 2021 there were 16 large active fires searing through Oregon according to the NIFC.

Air quality monitoring and measurement are critical for public health, says Yanelli Nunez, a postdoctoral research scientist at Columbia University’s Mailman School of Public Health.

She notes that robust studies have shown that air pollution contributes to lung cancer, chronic obstructive pulmonary disease, and lower respiratory infections, and even impacts mortality, pregnancy outcomes and cardiovascular disease.

Nunez works in an environmental health sciences laboratory with Marianthi-Anna Kioumourtzoglou. Their research has also found long-term exposure to air pollution can affect the nervous system and may influence functions such as memory or cognitive capabilities.

The scientists wrote in an e-mail to CNBC: “Americans living in poor air quality areas tend to be people of color or low-income communities. We are finally starting to pay more attention to these issues, which hopefully will lead to change. The air pollution composition is also changing.”

In one example, greenhouse gas emissions from transportation declined in New York City from 2014 to 2017, while commercial cooking emissions rose.

With increased wildfires, the scientists wrote, “The sources and composition of the air pollution mixture that we are experiencing could differently impact our health, so we need to better understand source-specific effects, especially for these newly prominent sources.”

While outdoor air quality is important, society doesn’t talk or do enough about indoor air quality, said Richard Corsi, UC Davis’ incoming dean of the college of engineering, currently a professor and dean at Portland State University.

Using pre-pandemic numbers, Corsi explained that the average American would spend almost 70 out of 79 years of their life domiciled inside of buildings. “Because we spend so much time indoors, even our exposure to pollutants of outdoor origin is dominated by what we breathe there, especially in our homes,” he said.

Pollutants of outdoor origin which come from the likes of internal combustion engine vehicles, photochemical smog, refineries and wildfires can get into homes and buildings when doors and windows are opened, when heat and air conditioning systems are used, or through other cracks in the building envelope.

Consumer apps and devices today don’t give users an absolute, precise measurement down to micrograms per cubic meter of a given pollutant, Corsi noted. But they’re very valuable for spotting trends and relative changes in air quality.

Sensors set up indoors can work well to check whether protective measures are working to improve the air inside of a house, school or other building.

Especially during wildfire season, Corsi said, some other simple actions that can protect or improve air quality indoors include: wet-mopping floors and wiping surfaces so pollutants don’t accumulate, using HEPA or high-efficiency particulate air filters, and increasing the MERV or minimum efficiency reporting value of filters in central air systems in a house.