Hydrogen sulfide, a common byproduct of oil and gas development, is an explosive gas, similar in toxicity to cyanide, that attacks the nervous system. Recognizable in trace amounts by its distinctive rotten egg odor and often referred to as "sour gas," it can be lethal.

The so-called "safe" exposure limits for hydrogen sulfide are based on the theoretical amount a healthy, 160-pound male worker could tolerate. The H2S occupational limit is 10 parts per million (ppm) over a portion of each workday.

In the real world, people do not react uniformly to H2S. Among the most vulnerable are adults with chronic illnesses, and children. Even someone with a cold has a weakened immune system, and consequently a lower tolerance for H2S exposure.

The Michigan Department of Community Health recommends an exposure limit for the general population of 0.01 to 1 ppm, although no standard is yet in place. In comparison, California has a public exposure limit of 0.03 ppm, and Elberta, Canada, has a limit of 0.02 ppm. Even at these levels, however, some people can experience headaches and other symptoms of H2S poisoning.

At very low levels H2S can deaden nerve endings, which means that an exposed person is unable to detect the rotten-egg odor to know that he or she is at serious risk. Exposure to H2S causes lung, eye, and skin irritation, nausea, light-headedness, and disorientation. As concentrations increase, so do the severity of the injuries, including unconsciousness and the risk of death.