Monitoring H2S to Meet New Exposure Standards

2010 ACGIH guidelines provide an impetus for companies to collect H2S monitor data, allowing them to evaluate and refine their safety and hygiene programs.

• By Rick Wanek
• Sep 01, 2011

Recently, the American Conference of Governmental Industrial Hygienists (ACGIH), a broadly recognized authority on the health effects of toxic gases, changed its recommended threshold limit values (TLVs) for airborne hydrogen sulfide (H2S) exposure. Previously, the ACGIH recommendation for an eight-hour time weighted average (TWA) exposure limit was a concentration of 10 parts per million and the 15-minute short-term exposure limit (STEL) was 15 ppm. The new recommendations for airborne H2S exposure are a TWA of 1 ppm and an STEL of 5 ppm. While compliance with these recommendations is not mandatory, they were developed from scientific data gathered by ACGIH during several years on the health effects of H2S exposure.

Many companies may feel compelled to use the new limits in their industrial safety and hygiene procedures. This makes sense from both a health and legal perspective. The possibility of a worker's suffering serious injury from H₂S exposure and a subsequent lawsuit could have a serious economic impact on a company. Still, there is the question of whether or not these new ACGIH limits are practical in terms of current monitoring and detection technology. Are electrochemical sensors available that can accurately detect an H2S concentration at some ceiling limit value that makes sense in terms of the ACGIH-specified TWA and STEL, without a lot of false alarms?

Health Effects of H2S Exposure
There is little doubt about the health effects of breathing air contaminated with H2S. In 1995, NIOSH updated and published the IDLH (Immediately Dangerous to Life or Health) value of 100 ppm. Although most people can smell very low concentrations of H2S, it is dangerous to assume odor provides adequate warning. At concentrations above the IDLH level, a person's sense of smell is quickly deadened. Table 1 lists bodily responses when breathing in various airborne concentrations of H2S.

Table 1. Physiological responses to various airborne concentrations of H2S
Air Concentration: Physiological Response
1,000-2,000 ppm: Loss of consciousness and possible death
100-1,000 ppm: Serious respiratory, central nervous, and cardiovascular system effects
150-200 ppm: Olfactory fatigue (sense of smell is significantly impaired)
100 ppm: Immediately Dangerous to Life and Health (IDLH concentration)
5-30 ppm: Moderate irritation of the eyes
5-10 ppm: Relatively minor metabolic changes in exercising individuals during short-term exposures
Less than 5 ppm: Metabolic changes observed in exercising individuals, but not clinically significant
5 ppm: Increase in anxiety symptoms (single exposure)
5 ppm: Start of the dose-response curve (short-term exposure)
0.032-0.02 ppm: Olfactory threshold (begin to smell)
Adapted from ACGIH and NIOSH sources

H2S Monitoring In The Past
In most cases where exposure to H2S is possible, company procedures call for personnel to wear portable detection devices and take appropriate precautions if an action point concentration is detected. The past practice of many companies has been to use electronic H2S detectors and monitor for conditions that exceed older ACGIH guidelines. Typically, they have monitored H2S using the more stringent operating exposure limit (OEL) -- i.e., a 10 ppm ceiling averaged over one minute. Using this procedure, H2S meters are typically set to alarm at 10 ppm and action is taken to reduce exposure, usually by leaving the area or by using SCBA equipment when air concentrations go above 10 ppm.

In Canada, for example, the Department of Government Services, Occupational Health and Safety Branch, is continuing to enforce 2009 ACIGH guidelines until issues surrounding availability and use of detection equipment is resolved. In the interim, industry is encouraged to consider using the 2010 ACGIH H2S guidelines when evaluating specific workplace situations and conditions and making risk management decisions.

The potential for personal injury lawsuits may compel industrial hygiene and safety professionals to routinely monitor, collect, and review H2S concentration data. This would allow them to determine whether a new action point concentration should be adopted based on 2010 ACGIH guidelines. Moreover, it would remove most of their doubts about whether or not current detection equipment has the sensitivity, resolution, and accuracy to support the new guidelines without a lot of false alarms.

Use of Current Monitoring Technology
Although H2S detection and monitoring practices vary by industry and the location of company operations (state, province, country, etc.), the prevailing methods in the workplace are detector tubes and instrumentation that uses electrochemical sensors. While some detectors can continuously collect gas concentration data from a fixed location in a monitored area, historically, few companies have actually built an archive of such data. If they did, this would allow them to establish ceiling limits in a quantitative manner, similar to the way ACGIH did in establishing its current H2S TWA and STEL values. The 2010 ACGIH guidelines provide an impetus for companies to collect H₂S monitor data, which would allow them to evaluate and refine their safety and hygiene programs.

Adopting the Latest Detection Technology
An electrochemical sensor is the most practical type of H2S detector because it responds in seconds to a gas exposure. Still, when selecting such a sensor, be sure it has the sensitivity and accuracy needed to support the new H2S guidelines.

Look for a sensor with:

• a response time of 15 seconds or less
• a lower detection limit (sensitivity) of at least 0.5 ppm; 0.1 ppm resolution (smallest detectable change) and an accuracy of ± 5% over its calibrated range of at least 0-100 ppm are ideal.
• an accuracy of ± 0.05 ppm at 1 ppm (± 5%) to meet the requirement of a reliable alarm at 1 ppm
• a built-in datalogging function for data collection and analysis
• an intrinsically safe design for use in areas where a combustible atmosphere may exist

In addition, a portable monitor designed to meet the new ACGIH H2S guidelines should have a low probability of false alarms. Generally, this requires a design with low temperature drift (typically, less than 0.1 ppm for the zero reading) and high selectivity for H2S in the presence of interfering gases, such as sulfur dioxide, nitrogen dioxide, and hydrocarbons. Reliable H2S measurements at sub-ppm levels and selectivity to discriminate H2S from interfering materials in the work environment are crucial elements of a monitoring device.

Incorporating H2S Monitoring Technology into Your Program

Industrial hygiene safety programs exist to create and maintain a safe, healthy workplace for employees, and the new ACGIH recommendations are no different. While these recommendations are expressed in terms of an eight-hour TWA and 15-minute STEL, most companies are likely to set action points in their safety and hygiene procedures based on instantaneous exposures or one-minute average ceiling limits, as read from an electronic sensor. With that type of procedure, the detector alarm level would probably be set somewhere between 1 ppm and 5 ppm and would require unprotected workers to immediately evacuate the area when the alarm goes off or don SCBA gear.

Using 1 ppm as a ceiling, for example, means the 2010 ACGIH OEL values of 1 ppm TWA and 5 ppm STEL could replace older values in existing procedures on H2S exposure. Advantages of this approach include simplicity, willing acceptance by workers, minimal training, easy documentation changes, and compliance with new OELs. Lower OELs require greater use of SCBA equipment, but that should not be viewed as a disadvantage.

Treating 1 ppm as a TWA is more complicated because it requires averaging detector readings over an eight-hour period. This could be done by equipping workers with a portable pump and detector tube over an entire shift, then sending the tube out to be read at an off-site lab. For some work areas, it may be practical to have an electronic detector installed in a fixed position and its data output connected to a laptop PC running software that computes a time-weighted TWA and provides an alarm output. However, due to the dangers of severe overexposure, personal portable H2S detectors with an alarm set at a 5 ppm or lower ceiling probably would still be required.

The 1 ppm TWA meets the requirements of the new guideline and should be acceptable to government inspectors. However, if only an eight-hour TWA is used, then overexposure is not detected until after it has occurred. Given the highly variable concentrations of H2S found in many workplace situations and the danger of collapse from overexposure, this is not likely to achieve broad acceptance by workers. To give them information needed to make rational decisions on protecting themselves, a personal portable H₂S meter is still required.

Important Features in Portable/Personal Monitoring Equipment

• Look for the ability for several sensors to be installed in the unit that can be selected from a list that includes CH₄, H2S, SO₂, CO, O₂, H2, NH₃, HCN, NO, NO₂, and others.
• Simple bump test station and a traceable bump test mode; bump testing every day is recommended to check sensor accuracy, stability, and response time and verify proper operation of the display and alarm functions.
• A variety of power supply options: Lithium alkaline or rechargeable NiMH battery supplies with charging module and charger are popular ones.
• Built-in event or data logger with an optional USB adapter and cable so that the personal monitor can output exposure data to a PC for analysis and reporting.

Conclusions
From the standpoint of available technology, there is no reason not to adopt the 2010 ACGIH guidelines for monitoring H2S exposure. Adopting the new guidelines and equipping workers with a personal/portable H2S meter will give them greater protection and, at the same time, help protect companies from costly personal injury lawsuits.

This article originally appeared in the September 2011 issue of Occupational Health & Safety.