With regard to DNA, weather conditions such as rain can wash away potential DNA on a firearm.

Does rain wash away DNA?

DNA traces on clothes of drowned bodies can provide important evidence for police investigations, especially in cases of suspected suicides or homicides. However, it is generally assumed that the water "erodes" a large part of the DNA depending especially on the exposure time.

Does anything destroy DNA?

Environmental factors, such as heat and humidity, can also accelerate the degradation of DNA. For example, wet or moist evidence that is packaged in plastic will provide a growth environment for bacteria that can destroy DNA evidence.