Hydrogen sulfide (H₂S), and related reduced sulfides, are produced when bacteria break down organic material such as plant or animal tissues where oxygen is limited (e.g., swamps, wetlands). It is not an unusual substance, and occurs in nature, as well as from some industrial sources. The World Health Organization (WHO) estimates that 75 to 250 million tons per year of H₂S are released from all land and ocean sources, including natural sources, industrial petroleum processing, geothermal energy production, tanneries, and paper mills of various types.

H₂S is easily detected by smell compared to other odors. While the reported detection threshold for H₂S in air for most people is in the range of 1 to 10 parts per billion (ppb), a very wide range from 0.5 to 300 ppb often is reported. Rotten eggs produce hydrogen sulfide and other reduced sulfur compounds when they decompose.

As described above, H₂S is generated as a gas from natural and industrial processes such as paper mills under normal circumstances.  It dissolves in water, but its vapor pressure causes it to leave water quickly. There are no federal or state drinking water standards for H₂S, and it is of health interest only in rare situations. If H₂S taste or odor is present in water supplies, it is treated by filtration or oxidation. Occasionally, H₂S is produced in homes by sulfate bacteria in electric hot water heaters.  H₂S is also formed as a natural product in our bodies, for example “morning breath” and intestinal gas.

H₂S occurs naturally, is used industrially to produce sulfuric acid and elemental sulfur and can be used to produce “heavy water” for nuclear plants. It has shown protective and beneficial characteristics in Alzheimer’s models and some other studies.

H₂S typically is regulated as an odor, not a health concern. The Agency for Toxic Substances and Disease Registry (ATSDR) set a Minimal Risk Level (MRL) for H₂S at 70 ppb to protect sensitive portions of our population, such as the elderly or young.  H₂S air levels below 70 ppb may be detectable by smell, but are not harmful to people or pets, even if exposures are common, or if brief, infrequent exceedances occur.

Exposure to concentrations of hydrogen sulfide greater than the MRL of 70 ppb in air for periods of time exceeding several weeks may cause transient and mild irritation of eyes, nose, and throat, which are reversible when exposure ceases. Such effects are irritated by the annoyance related to H₂S odor, which can initiate or intensify the effects. Individuals who are unable to smell H₂S do not experience such responses.

Short-term exposures to H₂S at concentrations of parts per million or higher (1 ppm = 1,000 ppb) have been reported to cause effects including nausea, headaches, disturbed equilibrium, neurobehavioral changes, olfactory paralysis, loss of consciousness and tremors. While some persistent effects have been reported in workers exposed to H₂S at high levels for long periods, effects from environmental exposures are reversible.

If you are outdoors when you smell H₂S, you may wish to go indoors temporarily to avoid the smell. Because it is a relatively common airborne substance, and often is associated with sewage treatment plants, sewage lift stations, decomposing vegetable matter, landfills and some industrial activities, it often is detectable near these facilities. If smells are localized indoors, ventilating the home typically is beneficial, assuming that it is not a continuing source in the home itself (e.g., hot water heater or water supply).

The workplace OSHA concentration for H₂S is 20,000 ppb (20 ppm), based on a 15-minute time-weighted average. Other regulatory agencies have developed guidelines for H₂S, including in occupational and environmental circumstances. There are no federal standards for air (other than for petroleum refining facilities), but protective guidelines have been recommended at the MRL of 70 ppb, which is set as the safe level for exposure periods less than or equal to 14 days.

In situations where people can move around freely, H₂S is almost never a health issue, though the odor of H₂S is easily detected. Protective equipment is not necessary except for some enclosed industrial operations.

The U.S. Bureau of Labor Statistics estimated fewer than 50 fatalities during the period from 2011 to 2017, approximately 8 per year. The cases typically are attributed to industrial accidents or entry into confined spaces containing high levels of H₂S.

There are many sources of information regarding H₂S, ranging from simple and straightforward to highly detailed and complex. These include:

ATSDR (Agency for Toxic Substances & Disease Registry). 2016a. Toxicological Profile for Hydrogen Sulfide and Carbonyl Sulfide. U.S. Department of Health and Human Services. Atlanta, GA.

ATSDR (Agency for Toxic Substances & Disease Registry). 2016b. Tox Guide for Hydrogen Sulfide (H₂S). U.S. Department of Health and Human Services, Public Health Service. Atlanta, GA.

ATSDR (Agency for Toxic Substances & Disease Registry). 2016c. Public health Statement: Hydrogen Sulfide (H₂S). U.S. Department of Health and Human Services, Public Health Service. Atlanta, GA.

California ARB (California Air Resources Board). 2021. Hydrogen Sulfide & Health. 

MN DOH (Minnesota Department of Health). 2019. Hydrogen Sulfide and Sulfur Bacteria in Well Water: Well Management Program. August 2019.