I'm getting some work done on my house and the guy doing the work wanted to test for lead paint. He scratched an area on a window sill, rubbed it with the spongy tip of a swab, and immediately declared it lead free cause it didn't change to the color of red. However, I just walked into the room three hours later and see the two spots he tested are both red. So what should I think of that?
I presume his test kit is designed to give an immediate result and, hours later, might be showing residue of lead from decades ago. Or perhaps just the breakdown of the chemicals in the test. I don't know.
EDIT: From looking around, one "instant" lead check swabber recommended by the EPA said theirs gives results in 30 seconds. We weren't in the room long but it was longer than 30 seconds.