While there are no regulatory exposure limits outside of the workplace for hydrogen sulfide (H2S), there are exposure guidelines to which any monitoring data will be compared. The Center for Disease Control and Prevention’s Agency for Toxic Substances and Disease Registry (ATSDR) has established an acute Minimal Risk Level (MRL) for H2S. A MRL is an estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse health effects over a specified duration of exposure.1
The Acute MRL for continuous exposure from 1 day to 14 days is 0.07 parts per million (ppm), which is equal to 70 parts per billion (ppb). The daily monitoring reports will show one day (24 hour) time-weighted averages in parts per billion (ppb).
Part per billion (ppb) equivalents are shown on this page because other data related to this investigation has been expressed in these units. It is critical that the units of measurement between monitoring results and the exposure guidelines match. For example, both should be in either parts per million (ppm) or parts per billion (ppb). It is easy to convert one to the other. 1 ppm = 1000 ppb, so to go from ppm to ppb, multiply by 1000; and to go from ppb to ppm, divide by 1000.
It is important to understand units of measurement when looking at monitoring results. A part per billion (ppb) is the equivalent of a penny in a ten million dollars. See Table 1. Below for a perspective.
The instruments being used for the monitoring done by DHEC can measure down to 1 part per billion (ppb) up to 10 parts per million (ppm).
The odor threshold range for hydrogen sulfide is 0.0005 to 0.3 ppm (0.5 to 300 ppb).
Additional exposure guidelines: Acute Exposure Guideline Levels (AEGLs) were developed by the National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances. These levels represent exposure limits for the general public for periods of time ranging from 10 minutes to 8 hours. Three levels – AEGL-1, AEGL-2 and AEGL-3 are established for each of five exposure time periods shown in the table below. Values are shown for concentration in both ppm and ppb.
Hydrogen Sulfide2
| Classification | 10 min | 30 min | 1 hour | 4 hour | 8 hour |
|---|---|---|---|---|---|
| AEGL-1 (Nondisabling) | 0.75 ppm 750 ppb | 0.60 ppm 600 ppb | 0.51 ppm 510 ppb | 0.36 ppm 360 ppb | 0.33 ppm 330 ppb |
| AEGL-2 (Disabling) | 41 ppm 41000 ppb | 32 ppm 32000 ppb | 27 ppm 27000 ppb | 20 ppm 20000 ppb | 17 ppm 17000 ppb |
| AEGL-3 | 76 ppm 76000 ppb | 59 ppm 59000 ppb | 50 ppm 50000 ppb | 37 ppm 37000 ppb | 31 ppm 31000 ppb |
1ATSDR Toxicological Profile on Hydrogen Sulfide: https://www.atsdr.cdc.gov/ToxProfiles/tp114-a.pdf
2AEGL for Hydrogen Sulfide: https://www.epa.gov/aegl/hydrogen-sulfide-results-aegl-program