Is the Apple Watch safe for environment?

Your mention of “intrinsically safe” suggests you are asking about certification for use in potentially explosive atmospheres and similar hazardous environments. Any approval for this under either US or European ATEX schemes must be clearly marked on the device and would also be referenced in the documentation. Apple does not mention approval for any of its products so you can be sure Apple Watch does not have any hazardous environment ratings. That doesn’t mean it is explicitly unsafe, simply that as a consumer product it has never been examined or tested in this respect.

See this ta

https://support.apple.com/en-gb/HT204508

Apple's Environmental Report on the Series 4 Apple Watch: https://www.apple.com/environment/pdf/products/watch/Apple_Watch_Series4_GPS_PER_sept2018.pdf

Ask Apple. I think they are the most environmentally concerned company in the world.