Gel nails- cancerous ?

I went to the nail salon and I was waiting for about 30-40 minutes so I was just getting a full white tip set. I sit down and it's just a regular acerlic nail then after everything is done he puts this fingernail polish over it he has never done this before and sits me under neath a uv light I didn't say anything thinking maybe this was something new. I got done and he says it will be 30 I didn't charge you for the gel. Well I knew because it was cancerous I never got it done. But how are they cancerous if it's just like a regular acerlic?? Thanks