Lovecraft grew up in age when a boy could have a complete chemistry laboratory in the cellar. For it he could acquire all sorts of hazardous chemicals, and from it he could pour all sorts of gaseous fumaroles, without causing a SWAT team to camp out on his front lawn…

I would give my mother and grandfather no peace till they had fitted me up a chemical laboratory in the basement of our home and there I dabbled in reagents and precipitates from March 1899 onwards, ploughing feverishly through such chemical primers as The Young Chemist” (Selected Letters II, p.109)

As a boy Lovecraft even had a small nuclear device. The cultural expectation that intelligent boys would safely play with home chemistry laboratories, and could even handle a few pinches of radioactive matter continued even into the 1950s. When a $50 toy chemistry kit could still come stocked with four different types of real uranium…

My essay on “The Colour out of Space”, in my book Historical Context 2 goes into more detail about the what Lovecraft could have known about radioactivity.