Actually, they can. You have to make sure the brand you are buying is decent. Burn one in a jar and make sure its not producing any matierial that could seep down into your ear.
Usualy, once in a while I asked my wifey to clean it for me. Somehow, she messed my left ear on the last one. I barely could hear nothing for two days. Went to the doctor, and he cleaned it well so fast!