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 our 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). [The first four reports were reported in ppm, but have been re-published in 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 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

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 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 - through July 5, 2022

General weather discussion through Tuesday:
A weak area of tropical low pressure will move up the South Carolina coast Friday night and Saturday. A trough of low pressure will linger over the foothills and Piedmont on Sunday, followed by a weak cold front passage Sunday night and Monday. The front will dissipate south of the region on Tuesday as high pressure builds in. There will be chances of showers and thunderstorms through the period, with the best chance being Sunday and Sunday night.

Specific discussion related to winds and odor in the vicinity of New Indy:

Friday night and Saturday:
Winds Friday night and Saturday will be light and variable much of the time, occasionally turning out of the south at speeds of 5 mph or less. Any odor plumes will mainly affect locations in the immediate vicinity of the facility, as well as communities a little ways to its north.  

Saturday night and Sunday:
Winds will again be light and variable or calm at times Saturday night and Sunday. During periods of light wind, areas immediately surrounding the facility will primarily be affected by any plumes of odor.

Sunday night and Independence Day:
Winds Sunday night are expected to become northeast at around 5 mph. Northeast winds will gradually become light and variable again on Independence Day.  Any odor plumes will primarily affect communities to the southwest of the facility Sunday night and Monday morning. By the afternoon hours, any odor will mainly affect locations close to the facility. 

Monday night and Tuesday:
Winds Monday night will be light and variable or calm, possibly becoming light southwest at speeds of 5 mph or less later at night. On Tuesday, winds are expected to be out of the southwest at speeds of 5 to 7 mph. Areas immediately surrounding the facility will primarily be affected by any odor plumes Monday night with communities to the northeast of the facility expected to be affected by any areas of odor on Tuesday.
The next update will be on Tuesday, July 5th (for Wednesday, July 6th).

DHEC Daily Reports

EPA Reports