National Grid ESO
Open data from Great Britain’s Electricity System Operator
Open air quality data.
OpenAQ API is a free Environment API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Welcome to the OpenAQ API documentation!
Details from docs.openaq.org.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Open data from Great Britain’s Electricity System Operator
Corporate carbon emissions disclosures (scope 1, 2 and 3), company-reported and estimated
Air quality of China
The OpenAQ API is provided by OpenAQ Docs (docs.openaq.org). Welcome to the OpenAQ API documentation!
Yes. The OpenAQ API is listed in the Environment 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.
Yes. The OpenAQ API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the OpenAQ API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The OpenAQ API is served over HTTPS, so it's safe to use in production.