https://www2.purpleair.com/products/list Pou mwen yo sanble tankou yon konpayi ki vann pwodwi ak kat yo se segondè. Yo menm fè piblisite bagay yo sou kat la tèt li.

“PurpleAir is built on the ideal of sharing data and by doing this, empowering individuals to effect change. All public PurpleAir data is available to download under various licenses from private, not for profit use to educational and commercial use. The common element between these uses is to share attribution.” (https://community.purpleair.com/t/about-the-data-category/70) – based on that maybe 4/5 blocks would be accurate. There do not seem to be any trackers or advertisements on the site.