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.
and 
for direct transmission and environmental transmission.
The dashed vertical lines indicate the level of 
above which 
becomes so large that no infection takes place (c.f. horizontal line in figure 7). Note that results for 
and 
are virtually indistinguishable and therefore the curves are on top of each other.
