Air Monitoring for Hydrogen Sulfide

Community Monitoring Update

Data has and will continue to drive decisions and activities related to the continued investigation into odors coming from the New Indy Catawba location. The EPA has assisted us by providing ten fixed monitors since May 13, 2021. All of the 24 hour averaged data is well below the Acute Minimal Risk Level (MRL) of 70 parts per billion. The continuous monitoring over this time period has helped to quantify (measure the concentration) of the odors reported by citizens.

On June 30, the EPA will be removing their monitors. Monitoring will continue in two ways:

  1. Under the EPA Order issued May 13, 2021, New Indy will conduct onsite and community monitoring for one year; and
  2. DHEC will be adding two continuous monitors, using a third to conduct unannounced co-located monitoring of New Indy monitors.

Monitors located closer to the facility provide more actionable data. The facility recently saw a higher concentration at one of their fence line monitors that led to the identification of a potential source of hydrogen sulfide. The data resulted in actions that greatly reduced the concentrations detected at the fence line, which most likely also reduced the concentrations in communities downwind.

The goal of both the EPA and DHEC Orders is for the site to continue to identify sources of hydrogen sulfide emissions and ways to reduce them. The data from monitors, especially those close to the facility, will continue to be used to measure increases or decreases in emissions related to specific activities on site.

Important data and information about activities on the site designed to reduce emissions impacting offsite locations can be found in the weekly reports provided by New Indy as well as in their Corrective Action Plan.

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: 
2AEGL for Hydrogen Sulfide:

Air Monitoring Results

The columns that are on the results document are explained here: 

Instrument Analyte ASTDR MRL Exceedance Number of Readings Number of Detections Concentration Range Period Average ASTDR MRL
Identifies the instrument at each location. Monitors are analyzing for hydrogen sulfide. Will indicate yes or no (compares Period Average column to ATSDR MRL column). Indicates how many readings the instrument  took during the period. Indicates the number of readings above the detection level of the instrument during the period. Indicates the range of readings on the instrument for the time period. Readings averaged over the 24 hour time period identified at the top of the table. The Acute ATSDR Minimum Risk Level (see definition above).

The red line on the graphs shows the ATSDR Acute Minimum Risk Level. It is possible that some readings during a period may exceed this level, but the comparison is to the concentration averaged over the 24 hour sampling period.

These are stationary monitors and wind direction is an important variable to consider. There will always be information on the prevailing wind during the sample period included in the attached reports.

Wind Forecast - through July 26, 2021

General weather pattern discussion (through Monday):
High pressure will move off the southeast coast on Saturday, with the Bermuda High reestablishing itself again for Sunday and Monday. Modestly lower humidity on Saturday will give way to seasonally hot and humid conditions thereafter. There will be a slight chance of an afternoon shower or thunderstorm on Saturday. No rain is anticipated on Sunday, with scattered showers and thunderstorms returning for Monday afternoon. 

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

Saturday, 24 July thru Monday 26 July:
Winds on Saturday will be out of the south at 5-10 mph, with winds on Sunday and Monday out of the southwest at similar speeds.  This wind flow will push any odor plumes in a north to northeast direction away from the facility. Winds will be gusty in and near any thunderstorms, with the wind direction becoming variable during these storms.    

Periods of light and variable winds are expected during the overnight hours and a few hours after sunrise each day, so areas immediately surrounding the facility could be affected by the odor during these times.

The next update will be on Monday, 26 July (for Tuesday, 27 July).

DHEC Daily Reports

EPA Daily Reports