Can you tell me how formaldehyde effect DNA cells in animals and humans?

I believe it doesn't. We pathologists spend our lives around formaldehyde without any apparent ill-effects unless we become allergic. There's a rare nose cancer for which it might act as a promoter. If any of us believed it actually damaged DNA in real-life situations, we wouldn't use it. The scares about formaldehyde may relate to the fact that it's made primarily by the far-right-wing Koch brothers.