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 below the table.  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 October 18, 2021

General weather pattern discussion (through Monday): A cold front will move into and across the state on Saturday. High pressure is expected to build eastward from the southern plains Sunday and Monday, and move overhead by early next week. The cold front will bring a wind shift this weekend with a much cooler airmass likely behind the front. The wind should range from south to southwest early in the weekend and then turn northwest as the cooler air arrives. 

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

Tonight through Saturday 16 Oct: Winds should be light tonight averaging south 3-5 mph. The wind will turn southwest and increase to 5-10 mph Saturday morning. The cold front is expected to cross the region Saturday afternoon bringing a west to northwest change in the winds with speeds of 10-15 mph, gusting to 20 mph. Odor is most likely to impact areas north and northeast of the facility tonight through Saturday morning, then the wind should push any odor east to southeast of the facility in the afternoon. 

Saturday night, Sunday 17 Oct and Monday 18 Oct: Northwest winds 5-10 mph will likely continue much of the period. These winds should turn west Monday afternoon and decrease to around 5 mph as high pressure builds more directly overhead. Any odor should be transported southeast of the facility through Monday morning, then east of the facility Monday afternoon.
The next update will be on Monday, 18 Oct (for Tuesday, 19 Oct). 

DHEC Daily Reports

EPA Reports