You're correct that certain types of plastic may not hold up well to heat or extreme cold. Plastics can vary in their heat resistance and stability, so it's important to choose the appropriate type of plastic for different applications.

When it comes to food storage and microwave use, it's generally recommended to use microwave-safe plastics that are specifically labeled as such. These plastics are designed to withstand the heat generated by microwave ovens without melting or releasing harmful chemicals. Look for containers that are labeled as "microwave-safe" or have a microwave-safe symbol (often a microwave icon) on the packaging.

It's important to note that even microwave-safe plastics can still potentially leach small amounts of chemicals into food when heated. To minimize any potential risks, you can consider using glass or ceramic containers for heating food in the microwave. These materials are generally safe and do not release any harmful substances into the food.

When it comes to extreme cold, most plastics are generally suitable for freezer use. However, some plastics may become brittle at very low temperatures, so it's a good idea to check the specific recommendations for the plastic containers you're using.

Ultimately, it's always a good idea to follow the manufacturer's instructions and guidelines for the safe use of plastic containers, especially when it comes to heat and cold exposure.