This is similar to linoleum block printing, except that the design is carved out with heat instead of cutting tools. I used a small 30 watt soldering iron with a separate voltage controller for precise temperature adjustment, so there is minimal burning of the material. Given good ventilation I think it is fairly safe to work with. I'll be going back for more experimenting, that's for sure. There is probably more air contamination from just looking at an automobile than there is from playing with this technique.
Unlike cut linoleum, the edges of the melted lines are slightly rounded. It is much easier to do gracefully curving lines with this technique than it is with linoleum block cutting. It also goes a lot faster; at about drawing speed.
If I really got into this, I would probably make a needle-tip soldering iron for fine details.
I used a non-toxic water based printing ink.
One nice thing about these rubber printing blocks is that you don't need a printing press. I did this print just by putting the block face down on the paper and pushing down on the block with my hands while rocking them around to spread the pressure out.