Laser labels are made from paper, adhesives and backing paper which are specified for laser printing. For example, the adhesive will be stable right up to 220 degrees (hottest lasers operate at 190 degrees; most are below 150 degrees). Laser paper also has some moisture content to help the toner to bond properly. If you put a sheet through a laser printer twice, then on the second print the toner may rub off. This is because the moisture has been reduced out on the first pass. It is also advisable to keep unused labels in their packaging in order to retain the water content.