I asked my doctor and he said it was fine. If you are dying your hair every week then there might be a concern but every 6 weeks or whatever is fine. Same with mani/pedis.
There are different opinions. I would just ask your doctor about this or research it. Unless you're asking opinions if people still dye their hair or not depending on the known research, there's nothing we can tell you that you can't find from Google.
I was told there was more concern years ago when there was more ammonia in the dye. I have a stylist who uses a permanent dye that doesn't have any at all. The grey coverage is great too. Used at Ulta salons. I don't know the name of the line, but I'll find out if you would like to know.
I am a colorist and am surrounded by haircolor all day long. Unless you are having your hair colored in a basement with no ventilation, there is nothing to worry about. Any good salon should be open and airy and fumes shouldn't be an issue. I've also heard people say they were told the color or bleach could seep into their skin through their head....a good colorist shouldn't be applying harsh chemicals to the scalp anyway. Color away!
Ventilation etc is necessary of course. However, if dying your hair BLACK there are some different chemicals in these dyes that should be avoided on your scalp as they can seep in and their effects are not well understood - particularly in the first trimester of development.
Re: Hair dye