The most popular hypothesis involved pheromones: airborne chemical messengers that are supposedly capable of resetting a female's internal clock.

The most harmful types of these airborne chemicals can burn your eyes, nose and throat, affect the nervous system, and cause headaches, nausea or cancer.

In this state they are sensitive to airborne chemicals and can detect food sources.

Olfaction as a sensory system brings awareness of the presence of airborne chemicals.

Common routes of exposure to waterborne pathogens include swallowing contaminated water, inhaling water droplets or airborne chemicals from the water, and direct physical contact with contaminated water.