Possible Air pollution, I'm not exactly sure but i think that might be the closest answer i found. I'm am truly sorry if this is not the answer! here is a little description about air pollution just in case.
Pollutants can be in the form of solid particles, liquid droplets, or gases. ... Usually, primary pollutants are substances directly emitted from a process, such as ash from a volcanic eruption, the carbon monoxide gas from a motor vehicle exhaust or sulfur dioxide released from factories.