The dishwasher subjects plastic items to a combination of high temperature, mechanical agitation, and alkaline detergent that dramatically accelerates chemical leaching and physical degradation. A study published in Environmental Science and Technology in 2023 found that dishwashing can release millions of microplastic particles per wash from polypropylene containers, and increases the leaching of bisphenol compounds and phthalates from plastic food storage items by a factor of several hundred compared to hand washing.
The mechanism: heat and detergent
The two main processes are thermal degradation and detergent extraction. At dishwasher temperatures (typically 65 to 75 degrees Celsius), plastic polymer chains undergo accelerated breakdown. Chemical additives, including plasticisers, stabilisers, and flame retardants, are liberated from the polymer matrix as the physical structure degrades. Detergents, which are strong alkaline solutions, are solvents for many of these additives, actively extracting them from the plastic surface and into the wash water. Food residue on the plastic surface can also act as a vehicle for transport: some contaminants dissolve preferentially into fatty or acidic food residue.
The research
A 2023 University of Bayreuth study focused specifically on polypropylene food containers and found that dishwashing released approximately 4 million microplastic particles per wash from a single container. The particles were in the sub-micron range, meaning they are nanoplastics capable of crossing cell membranes. A separate study in the same year found that bisphenol leaching from polycarbonate bottles increased by 200 to 500 times when bottles were dishwashed compared to hand-washed in cool water. The turbidity cycles and drying phase (which can use higher heat) both contribute to damage.
What is safe to dishwash
Glass, stainless steel, and ceramic items are safe to dishwash and do not leach or shed in the process. Cast iron and carbon steel cookware should not be dishwashed (it strips seasoning), but for chemical safety reasons rather than contamination reasons. The practical rule is to reserve the dishwasher for non-plastic items and hand-wash any plastic items you use for food in cool or warm water with standard dish soap. If you are transitioning away from plastic food storage, the dishwasher constraint is an additional reason to do so: the containers that most need to be replaced are exactly the ones most damaged by dishwashing.