Since Ziplock bags are made of thin plastic, they are prone to melting. At 195 degrees Fahrenheit, Ziplocks will begin to soften and warp. If exposed to this heat, it’s best to throw them away rather than reuse them. If exposed to higher temperatures, especially water boiling at 212 degrees Fahrenheit, Ziplocks will melt completely. When resealable bags melt, they ruin food, release harmful toxins, and can harm your skin.