Usually, threading, along with waxing is not particularly done under sanitary conditions which is why, waxing is becoming regulated in some states where infection control is being monitored more closely as a result of infection. Eventually, with threading becoming more popular, that too might become regulated.
If you do have threading done, make sure that the thread does not go into the practitioners mouth and that gloves are worn. Your face should be sanitized before and after.
Which is better? Threading or plucking? Threading on the face requires the help of a threader while plucking can be done by yourself. Threading removes lots of fine hairs simultaneously while plucking is done on single hairs.
Should you decide to have threading done, make sure that the threader is at least a licensed cosmetologist or licensed esthetician where they are tested and inspected by the state for sanitary conditions.