Google now will show air quality and pollution data in the search results. If you search for [air quality], [air pollution] and/or qualifiers with city qualifiers, such as [air quality in nyc], Google will show a rating of the air quality in that city.
Here is a screen shot of the NYC example:
And for Beijing, where the air quality rated as “unhealthy”:
This works both on desktop and mobile search on Google.com.
The source of the data is from a company named BreezoMeter, and if you click through on the company name in the footer of the answer, it shows more details about the air quality in that location.