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.
Time intervals evaluated in the included studies. The blue dotted line indicates the system interval defined by the Aarhus statement. Studies that included any aspect of this system interval were included, even if the interval commenced before the system interval defined here.
