No. The rays are not harmful. No need to worry about it.

It is sticked with composite which is cured with curing light which is harmless Also u should make it remove as its been 3 years

Permanent retainers is placed at back of teeth using tooth colored composite. For this composite to set completely and gain it's strength(to become hard) requires UV light. As per your query regarding the harmful effect of this light, amount of exposure is very less. This amount of exposure will not cause any particular type of cancer.

Hello dear I hope you are fine Well we use composite material to set the permanent retainers and to set (to gain strength) we need uv light of particular wavelength which has very less harmful effects .