There are an estimated 357 million firearms in the United States.
Source - https://www.washingtonpost.com/news/wonk/wp/2015/10/05/guns-in-the-united-states-one-for-every-man-woman-and-child-and-then-some/
There are 91 deaths and 200 non-lethal injuries caused by those guns every day.
Source - https://everytownresearch.org/gun-violence-by-the-numbers/
If we define each firearm-related death or injury as an "adverse outcome", those numbers yield a daily rate of 8.15 x 10-7 ... or about one in a million.
When deciding how much of a pollutant the public can be exposed to, environmental agencies often base the limits on a one in a million chance of an adverse outcome (such as cancer).
So, why is one in a million good enough for pollutants but not good enough for firearms?