Fig. 1.
Number of countries with air quality standard (AQS), with comparable standard, without standard and with unknown status regarding regulation of ambient air pollutants for long-term and shorter averaging times (up to 24-h)
Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.