Is laser engraving on ABS plastic harmful if you smell it?

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Of course no. Check the wikipedia below first:

ABS is stable to decomposition under normal use and polymer processing conditions with exposure to carcinogens well below workplace exposure limits. However, at higher temperatures (400 °C) ABS can decompose into its constituents: butadiene (carcinogenic to humans), acrylonitrile (possibly carcinogenic to humans), and styrene.

Generally speaking, if you want to laser engrave on ABS plastic, the power will reach at 400mW, therefore, it's harmful to your physical body.