Hydrogen sulfide (H₂S) occurs naturally. 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. The fact that you are smelling an odor does not mean that it is causing you harm. 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.

H₂S is not similar to carbon monoxide, which is a completely odorless gas. Generally, it is not possible to be harmed by H₂S if you do not smell it. Even when the odor is detectable, in situations where people can move around freely, H₂S is almost never a health issue. In other words, even if you do smell H₂S, it does not mean that what you are smelling is harmful. Most of the time it is not.

H₂S is a gas 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 protective and beneficial characteristics in Alzheimer’s models and some other studies.

H₂S typically is addressed 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. The 70 ppb MRL does not mean that levels above 70 ppb necessarily are harmful. Rather, it is a conservative guideline for sensitive members of the population.

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. 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 occasionally cause nausea, headaches, disturbed equilibrium, neurobehavioral changes, olfactory paralysis, loss of consciousness, and tremors, depending on the levels. 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 there is not a continuing source in the home itself (e.g., hot water heater or water supply).

Air filtration devices are available for residential applications and would be expected to help dissipate H₂S odors, both existing and new.

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 the safe level for exposures of less than or equal to 14 days.

H₂S is not regulated by federal law according to air concentrations. There are Clean Air Act requirements related to H₂S release prevention and planning. Therefore, H₂S release is regulated for reporting and planning purposes beyond threshold quantities, but not by air concentration. There are also federal Permissible Exposure Limits (PELs) that apply in the workplace, but not outdoor air.

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.