For operations utilizing PEM, there are no toxic materials released to the environment. The materials are recyclable. For liquid potassium hydroxide electrolyzers, post treatment of waste electrolyte is needed.
Oregon electrolyzer start-up manufacturer HydroStar USA responds to this question relating to their technology:
Our manufacturing processes are simple, standard machining like drilling, welding, and other machining processes as well as either 3D printing or injection molding of plastic parts. Our devices can use recycled 304 Stainless Steel reducing global material waste. The manufacturing site can be setup regionally thus reducing the logistics issues and significant transportation costs. Our electrolyte is benign and therefore does not have any issues when used or decommissioned. Our devices are largely made up of 304 SS tubing and 3D extruded plastic parts, all which can be recycled. The only potential issues of potential releases or toxic materials would be in the actual recycling process.