Are black widows harmful to the enviorment

Yes

Explanation:

Although black widow spiders can be dangerous to humans, they are also an important part of our ecosystem, and should not be killed when they are encountered. Plus, black widow spiders will only bite humans if they are grabbed or if their web is disturbed: since black widow spiders usually build their webs in out-of-the-way places (not in the middle of a trail, for instance) they are rarely a threat to people. Learn to recognize and avoid the black widow spider instead of killing it

if you mean hydrogen and oxygen then water is the product