Air Monitoring for Hydrogen Sulfide

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. 

Hydrogen Sulfide Units of Measurement
Table 1

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: 
2AEGL for Hydrogen Sulfide:

How do I Interpret the Daily Reports?

At the top of each daily report you will find the date and the 24-hour time period for each report (midnight to midnight). 

New Indy - Air Monitoring Table Example

Notes about the monitoring period will be put at the top of the page. For example if a monitor failed (could be a battery issue or an issue with a sensor) or if an error was made in a report, this section will contain an explanation of the correction. 

A map with locations of the facility as well as monitor locations is also part of each report. There will be a description of the prevailing wind direction during the monitoring period just before the charts. 

Monitors operated by DHEC take a reading every 30 seconds - that is the Number of Readings. A detect is defined as a measurable concentration during one of those 30 second readings.  There are columns in the report for number of readings and number of detections. 

The graph below shows a peak around 1:30 AM at ~50 parts per billion.  The 30-minute average level at which health effects are expected is 600 ppb (0.6 ppm) (See AEGL-1 description on the monitoring landing page.)  The 70-ppb red line is not a health standard but is a level that indicates further investigatory actions are warranted. The peak concentration recorded about 7PM exceeded the level of the 70 ppb Standard, but for less than an hour. 

New Indy - Air Monitoring Graph Example

The red line is the 24-hour exposure guideline to which monitoring results are compared.  Exposure guidelines have two components, concentration AND time. See the Acute Exposure Guideline table on the air monitoring page for exposure guidelines over shorter time periods. 

While we have seen brief periods of time with concentrations above the red line at several locations, we have not come close to exceeding the guideline over the time period used for comparison. The 70 ppb is an acute Minimum Risk Level (MRL) set by the Centers for Disease Control and Prevention as a level that warrants additional investigation should it be exceeded.

Wind Forecast

The daily Wind/Odor Forecast will be discontinued after the weekend forecast provided Friday, December 11. An authoritative hourly forecast is available from the National Weather Service for the area of the facility at this link

DHEC Daily Reports

The current onsite monitoring reported by the facility and boundary monitoring conducted by DHEC have documented the significant reduction in concentrations of Hydrogen Sulfide (H2S) near the facility and the continued reduction in intensity, frequency, and duration of H2S concentrations near the facility. Since September,2021, there haven’t been any instantaneous detections of H2S at any of DHEC’s continuous monitors at the facility boundary above 10% of the level of the most protective Acute Exposure Guideline (AEGL). This doesn’t mean there will not be odors detected offsite; it means the concentrations are consistently well below published health and safety guidelines. 

Since monitored concentrations have been well below these established guidelines and New Indy must continue to report results from their current monitoring and the additional monitoring required by the Order, DHEC will suspend facility boundary monitoring and reporting in mid-August, 2023 unless there is a change in activity at the facility. DHEC’s monitoring may resume if there is an increase in on-site monitored concentrations or a significant change in operations associated with potential odor sources. 

EPA Reports