According to the North Dakota State University (NDSU) Agriculture Communication, some bags are made from plastic that is too thin to boil without melting. They recommend calling the manufacturers of the plastic bag and asking them what the softening point is. NDSU reports that Ziploc brand bags have a softening point of 195 degrees, which means they would melt at boiling point, 212 degrees. This will ruin both the food and the dishes the bags are in.
When plastic is boiled, the chemicals used to produce it can leach into the food being prepared due to a high temperature. Common chemicals in plastic include BPA and phthalates. Harvard Medical School says that high-fat foods are especially susceptible. NDSU adds that some plastic bags will leach toxic fumes from the ink, glue and recycled materials used in making them.
The Food Domain says that when water is boiled in a bag it can cause bubbles to form that could burst when the bag is moved. Even if the water is outside the bag, the bag itself can still burst open when temperatures get very high. If either of these two things happen, the cooks may get burns on their face or hands. Adding salt or other spices can produce the same effect because it disturbs the bubbles that have formed, causing them to erupt. NDSU also reports that the bag can ignite, producing a fire and subsequent burns to a person standing nearby. This is most common when a microwave oven is being used.