Recently lowered exposure limit guidelines for H2S, SO2 and NO2 have forced many instrument users to revisit where to set the alarms in their atmospheric monitors. Exposure limits like the OSHA PEL, NIOSH REL and ACGIH® TLV® provide exposure limits, which if exceeded, may lead to immediate or long-term harm.

Workers should be out of the area before rather than after the concentration of toxic gas exceeds the hazardous condition threshold. Unfortunately, the PEL, REL and TLV® are not always in agreement.

OSHA regulations use the term Permissible Exposure Limit (PEL) to define the maximum concentration of a listed contaminant to which an unprotected worker may be exposed as an 8-hour time-weighted average (TWA) during the course of his workplace duties. Exposure limits for gases and vapors are usually given in units of parts-per-million (ppm). Limits for mists, fume and particulate solids are expressed in units of mg/m3. In addition to the 8-hour TWA PEL, Ceiling (C), Short-Term Exposure Limit (STEL), and peak exposure limits are also specified for some airborne hazards.

OSHA 8-hour TWA PEL and C values are given for some airborne contaminants in Subpart Z (Section 1910.1000), and in substance-specific standards (e.g., methylene chloride, 1910.1052 and benzene, 1910.1028).

NIOSH develops and periodically revises recommended exposure limits (RELs) for hazardous substances or conditions in the workplace. The OSHA PELs listed in Subpart Z are the same values originally promulgated in 1971. Because NIOSH RELs are periodically reviewed and updated, they tend to be more conservative than the OSHA exposure limits. Many states with approved occupational safety and health plans use the NIOSH RELs.       

What are TLVs®?

ACGIH TLVs® are not expressly developed for use as legal standards, but they are frequently incorporated by reference into state, federal and many international regulations. They may also be cited or incorporated by the National Fire Protection Association (NFPA) or the American National Standards Institute (ANSI). Given the potential for lawsuits, many employers base their corporate health and safety programs on conservative applicable recognized standards. Since ACGIH recommendations are frequently more conservative than OSHA PELs and NIOSH RELs, many programs, especially the programs of multinational or prominent corporations, use the ACGIH TLVs®.

How do instruments determine when to go into alarm?

Low and High alarms are sometimes also referred to as Ceiling or Peak alarms. Low and High alarms are based on actual, real-time measured values. The alarm is activated the moment the measured reading exceeds the setting.

STEL alarm calculations are generally based on a 15-minute time weighted average. Once the duration of the monitoring session exceeds 15 minutes, the STEL calculation is based on the average concentration over the most recent 15-minute interval. The STEL alarm is sounded whenever the most recent 15-minute interval average exceeds the alarm setting.

TWA calculations are generally based on an 8-hour time weighted average (TWA). When the instrument is initially turned on, the microprocessor calculates the TWA based on the actual concentration measured projected over a full 8-hour period. Time not measured is projected as zero exposure in most jurisdictions. Thus, for a worker exposed to 100 ppm of contaminant for four hours, the 8-hour TWA would be equal to only 50 ppm at that time. At the end of a full eight hours at 100 ppm, the 8-hour TWA would be equal to 100 ppm.

The TWA alarm is sounded whenever the projected or the elapsed exposure exceeds the alarm setting.

An important aspect of the TWA concept is that concentrations above the limit are permitted as long as they are balanced by an equal amount of time that is spent below the limit. Of course, at no time can the concentration exceed whatever other limits are specified.

The ACGIH® provides additional guidance for exposures that exceed the TLV®. According to the ACGIH®, exposure above the TWA up to the STEL should be less than 15 minutes, should not occur more than four times per day; and any 15-minute interval which exceeds the TWA limit should be separated by at least one hour from the next.

A complicating factor is that for many gases the TLV® provides only a single part definition. For instance, the 2012 TLV® for NO2 is a single-part recommendation that consists of an 8-hour TWA limit of 0.2 ppm. Although the NO2 TLV® does not include a STEL or a Ceiling limit, it is still very important to limit the maximum instantaneous exposure.

The ACGIH® “Excursion Limit” applies to those TLVs® that do not have a STEL. According to the ACGIH®, “Excursions in worker exposure levels may exceed 3 times the TLV-TWA for no more than a total of 30 minutes during a workday, and under no circumstances should they exceed 5 times the TLV-TWA, provided that the TLV-TWA is not exceeded.”1

Using the Excursion Limit concept, setting the Low instantaneous alarm at 0.6 ppm (= 3 times the TWA), the High alarm at 1.0 ppm ( = 5 times the TWA), and the TWA alarm at 0.2 ppm would meet both the spirit and letter of the TLV®.

H2S exposure limits

H2S is an extremely common workplace contaminant associated with microbial decomposition. It is a leading cause of injury and death in the oil industry, during confined space entry, and in many other workplace settings.

The federal OSHA PEL for H2S is quite different from, and less conservative, than the NIOSH REL and ACGIH® TLV®. The OSHA PEL for H2S does not include a TWA or STEL, but stipulates that the acceptable Ceiling (C) concentration is 20 ppm, that the maximum peak above the Ceiling (C) concentration is 50 ppm, and that the maximum duration of exposures above 20 ppm are limited to no more than 10 minutes once per eight hour shift when no other measurable exposures occur during the shift.

Be aware of specific activities that may impose additional monitoring requirements and/or take action thresholds. For instance, OSHA 1910.146, “Permit-required confined spaces,” Appendix D provides examples of a Confined Space Pre-Entry Check List and Entry Permit. The examples reference 10 ppm H2S as the maximum permitted concentration during pre-entry testing (after isolation and ventilation), and 10 ppm (TWA) and 15 ppm (STEL) as the maximum concentrations permitted during an entry.

The 2010 TLV® for H2S has caused much concern among instrument users. The old (2009) TLV®, as well as the current NIOSH REL, reference the same 15.0 ppm STEL and 10.0 ppm TWA limits.

The latest 2010 TLV® comprises a two-part definition, with a 15-minute STEL of 5.0 ppm, and an 8-hour TWA of 1.0 ppm. Where to set the instantaneous Low and High alarms is a particular concern for many instrument users. The fear is that if the instantaneous low alarm is set at the 1.0 ppm TLV®-TWA, frequent alarms due to momentary spikes above 1.0 ppm might cause workers to lose faith in the instrument, or cease taking the correct protective measures.

For instrument users who are unaffected by the new TLV®, the tendency has been to stick with the old alarm settings. For customers affected by the TLV®, many users find the following settings meet the spirit and requirements of the 2010 TLV limit: Low = 3.0 ppm; High = 5.0 ppm; STEL = 5.0 ppm; TWA = 1.0 ppm      

Are H2S sensors capable of measuring at the new TLV® limits?

The answer is “Yes” BUT with qualifications. Not all sensor designs are equally optimized along all performance dimensions (e.g. cost, life, size, response to interfering contaminants, sensitivity, AND ACCURACY!)

For an H2S sensor to be usable with one or more of the alarms set at the 1.0 ppm limit, it needs to be capable of providing low concentration readings with at least ± 0.2 ppm resolution. Some H2S sensors are easily capable of providing readings with ± 0.1 ppm or ± 0.2 ppm resolution; while others are limited to ± 1.0 ppm resolution. In general, dual channel “COSH” sensors used to simultaneously measure both CO and H2S have a smaller measurement signal than substance-specific single-channel H2S sensors. Some are capable of being used at the 1.0 ppm limit; some are not.

Also, the instrument programming (firmware) must permit setting the alarms at the desired concentration. Depending on the instrument, it may be necessary to update the firmware or even replace an older instrument with a newer model. Instrument users should consult with the manufacturer.          

SO2 exposure limits

The new TLV®-STEL is an extremely challenging limit. Existing instruments may or may not be able to perform at the new limit. Once again, for instrument users who are unaffected by the new TLV®, the tendency has been to stick with alarm settings at the old NIOSH and ACGIH® limits, (Low = 2.0 ppm, High = 5.0 ppm, STEL = 5.0 ppm, TWA = 2.0 ppm).

For customers affected by the TLV®, many users find the following settings meet the spirit and requirements of the 2010 TLV® limit: Low = 0.75 ppm; High = 1.25 ppm; STEL = 0.25 ppm; TWA = 0.25 ppm.