The safest response is to move away from the area, keep children and pets away, and contact emergency or hazardous-material responders. Fire departments are trained to identify and safely contain potentially dangerous substances.
Possible explanations could include a chemical spill, unusual biological material such as slime mold or insect larvae, or a badly decomposed animal. However, unusual smells combined with physical symptoms should always be treated seriously.
Do not attempt to clean, move, burn, or dispose of the substance yourself. Observe only from a safe distance, note its approximate color and size, and let trained professionals determine what it is and whether it poses a health risk.
