The safety of using certain plastics in food preparation and serving came under scrutiny in recent years, and the topic has remained controversial. If you know which type of plastic cup you have and how to wash it safely, you'll have few concerns about loading cups into your dishwasher.
The BPA Question
Manufacturers make a variety of types of plastic, with only a handful appearing in food containers, including drinking cups. Those containing a chemical called bisphenol A -- or BPA -- receive the most scrutiny over controversial concerns that it poses a health hazard, especially to children. The chemical is a main component of polycarbonate, a durable plastic that shows up in many drinking cups, among other items.
Heat and strong dish-washing detergent both can degrade most plastics over time, especially those containing BPA. The Washington State Health Department, therefore, recommends not putting plastic cups containing the chemical in a dishwasher, especially when using abrasive soap. Dishwasher manufacturers also discourage putting plastic cups made for a single use in the machine to avoid health issues or melting of the container. Other plastics may stand up to washing in the machine, but check recommendations from the manufacturer and put all plastics in the top rack.
Detecting the Type
Turn every plastic cup or container over and check the recycling code for a clue about whether you can safely wash it in the high heat of a dishwasher. Those with the code PETE or PET or code No. 1 can break down over repeated washings in hot water and strong detergent, and may not stay safe. Those with recycling code No. 5 generally contain BPA, the chemical that may leach into food and beverages, and those with numbers 7 or 3 may contain the chemical. Those marked with codes No. 6 -- normally plastic foam -- cannot withstand heat or strong detergent and should not be washed in the dishwasher or reused.
The use of plastics in food service and packaging, including drinking cups, remains controversial. Most manufacturers willingly stopped using BPA in the face of concerns over its possible health risks. Older pieces, however, likely still contain it, so carefully monitor plastics for damage -- and discard any that cause you to suspect their safety. Keep those that cannot handle the heat out of the dishwasher.