Real Time Air Quality Monitoring.
Purple Air API is a free Science & Math API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Real-time air quality monitoring with PurpleAir® sensors. Monitor, configure, and manage your sensors for accurate PM2.5, PM10, temperature, and humidity data.
Details from www2.purpleair.com.

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The Purple Air API is provided by PurpleAir (www2.purpleair.com). Real-time air quality monitoring with PurpleAir® sensors. Monitor, configure, and manage your sensors for accurate PM2.5, PM10, temperature, and humidity data.
Yes. The Purple Air API is listed in the Science & Math category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Purple Air API requires no authentication, so you can start making requests without signing up.
CORS support for the Purple Air API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Purple Air API is served over HTTPS, so it's safe to use in production.