What would you like to ask?im looking for a 70's or 80's film where kids finger nails turn black after passing?

im looking for a 70's or 80's film where kids finger nails turn black after passing through a fog on a school bus?What would you like to ask?im looking for a 70's or 80's film where kids finger nails turn black after passing?
...oh, you have to be thinking of the cheezy 1980 horror cheapie, entitled ';The Children';...