Note: All exposure limits cited in this article are current as of 28 January 2013

Ultimately, the entire matter of where to set the instantaneous concentration alarm(s) is tied into what is expected by the regulatory agency. In the United States, for most workplace environments, it is the Occupational Health and Safety Administration (OSHA).

OSHA has published Permissible Exposure Limits, or PEL’s, for many toxic substances. Three types of PEL’s are defined, although not for every substance.

  • Most common is the 8-hour time weighted average (TWA) value, referring to the maximum daily exposure, based on an 8-hour day/40-hour workweek. Note that the only practical way to document compliance with a TWA standard is via a data acquisition/archiving/reporting system, such as Interscan’s Arc-Max®.
  • For some airborne contaminants, an “excursion value” is set, generally referring to the maximum allowable concentration averaged over a particular 15-minute time period. Limits on the number of these excursions during an 8-hour workday, as well as a mandatory time interval between such exposures, may also be set. This parameter is sometimes called a Short-Term Exposure Limit (STEL).
  • In certain cases a “ceiling value” is designated, as the instantaneous concentration which should never be exceeded during the workday. A provision may apply in some cases whereby if instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time over a working day. To add to the confusion, occasionally a single value is promulgated as a “STEL/ceiling.”

One more term is used: “Action Level.” This is usually one-half of the allowable 8-hour TWA for the substance in question. The only significance of the action level is as a benchmark during the initial screening of a workplace. If concentration levels of the chemical in question are greater than or equal to this action level, then regular monitoring will have to be done. For certain compounds, such as ethylene oxide, OSHA has formulated detailed guidelines on the initial monitoring process.

Note that there is no logical reason to set an instantaneous alarm for a toxic gas monitoring system at the action level. This number is already one-half of a value that is allowable for an 8-hour time-weighted average exposure. In fact, setting alarms at the action level, unless there is a purpose behind it that has nothing to do with regulatory compliance, is to be avoided, as it tends to upset personnel needlessly, and may require more frequent instrument maintenance and calibration.

OSHA’s PEL’s are derived from many sources, including the National Institute of Occupational Safety and Health (NIOSH), the American Conference of Governmental Industrial Hygienists (ACGIH), and the American National Standards Institute (ANSI). NIOSH and ACGIH also publish standards for airborne contaminants, that are not always in harmony with the OSHA regulations, but are often helpful, especially in matters that OSHA does not address.

The instrument user is well-advised to make himself familiar with the applicable regulations. Of course, we at Interscan can provide guidance. Please contact our sales department for any applications assistance.

Now, let’s consider a few specific examples.

Carbon Monoxide (CO)

OSHA specifies an 8-hour TWA of 50 ppm. Setting the system alarm at 50 is not recommended, unless the workplace concentration is always teetering just below this level, and compliance with the 50 ppm standard is truly an issue.

Although OSHA does not define a STEL or ceiling value for CO, NIOSH does, and it is 200 ppm. Absent the condition of meeting the 50 ppm standard, a more prudent course would be to set alarm-1 (the warning alarm level) at 100 ppm and alarm-2 (the danger alarm level) at 200 ppm.

All Interscan continuous monitoring systems are provided with at least two adjustable alarm levels, and our portable units are equipped with a single adjustable alarm set point. In this example, a single alarm-equipped unit should be set between 100 and 200, so a logical choice would be 150 ppm.

Chlorine (Cl2)

OSHA does not specify an 8-hour TWA, but denotes a ceiling value of 1 ppm. ACGIH lists an 8-hour TWA of 0.5 ppm. Here, there is but a 0.5 ppm difference between ACGIH’s TWA and OSHA’s ceiling, which is probably why OSHA does not bother with a TWA. In this case, set points of 0.5 ppm for alarm-1 and 1 ppm for alarm-2 would be a reasonable idea. A single-alarm equipped unit should be set somewhere between 0.5 and 1, probably closer to 0.5.

Ethylene oxide (EtO)

OSHA calls out an 8-hour TWA of 1 ppm, with a STEL of 5 ppm. ACGIH has no STEL, but concurs with the 1 ppm TWA. We would recommend settings of 2 ppm (or 2.5 ppm) and 5 ppm, for alarm-1 and alarm-2, respectively. A single-alarm equipped unit should be set at 2 or 3 ppm.

There is no conceivable reason to set any alarm at the action level of 0.5 ppm, since this value is of no significance once monitoring is formally underway.

Formaldehyde (HCHO)

OSHA specifies an 8-hour TWA of 0.75 ppm, and a STEL of 2 ppm. ACGIH defines a STEL/ceiling of 0.3, with no 8-hour TWA, and NIOSH posts an 8-hour TWA of 0.016 ppm. Inasmuch as the U.S. Environmental Protection Agency’s Integrated Risk Information System (IRIS) classifies formaldehyde as a probable human carcinogen, the ultra-low numbers proffered by ACGIH and NIOSH surely reinforce the notion that exposure should be as low as possible. American workplaces are legally required to meet the OSHA limits. Still, if a workplace can be kept at lower levels, so much the better.

Our recommendation would be for settings of 1 ppm and 2 ppm, for alarm-1 and alarm-2, respectively, keeping a special eye on how close the TWA is getting to 0.75 ppm. A single alarm unit should be set to 1 ppm, again keeping a special eye on how close the TWA is getting to 0.75 ppm. For those occupancies that can be better controlled, alarm settings can be put lower, but more instrument maintenance may be required.

Hydrogen chloride (HCl)

No American agency specifies a time-weighted average exposure recommendation. Rather, only ceiling values are proffered. ACGIH sets theirs at 2 ppm, while OSHA and NIOSH have theirs at 5 ppm. The International Agency for Research on Cancer (IARC) denotes HCl as “IARC-3: Unclassifiable as to carcinogenicity in humans.” Similarly, the ACGIH calls it “Not classifiable as a human carcinogen.” Typically, this designation is used for compounds that “could” be carcinogenic, but of which there is insufficient data to draw this conclusion.

While the OSHA level has the force of law in the United States, prudence would dictate a nod to the lower ACGIH concentration. Thus, we would recommend that a single alarm unit be set to 2.5 or 3.0 ppm. A two-alarm unit should have alarm-1 at 1.9 ppm, and alarm-2 at 4.0 or 4.5 ppm.

Hydrogen sulfide (H2S)

OSHA does not specify an 8-hour TWA, but instead defines a ceiling concentration of 20 ppm. An additional provision allows an “acceptable maximum peak above the acceptable ceiling concentration for an 8-hour shift” of 50 ppm, for a maximum duration of 10 minutes once, and only if no other measured exposure (presumably above the ceiling value) occurs.  ACGIH lists a TWA of 1 ppm, and a STEL of 5 ppm.  NIOSH sets a 10-minute ceiling of 10 ppm.

Given the disharmony between OSHA and ACGIH, prudence would favor a bias toward lower alarm levels, although not necessarily in total keeping with ACGIH.

Clearly, the OSHA regulations were written to respond to differing situations faced by various industries that have hydrogen sulfide exposure. For those that actually need the 50 ppm loophole, alarm-1 should be set at 20, and alarm-2 should be set at 50. For those operating at lower levels, settings of 10 and 20 respectively would be advised.

Settings for a single alarm unit would be problematical for the high concentration scenario, while a setting of 20 ppm would suffice for the lower level applications.  In light of ACGIH and NIOSH, this setting could be reduced to 10 ppm, or even lower.

Sulfur dioxide (SO2)

OSHA has published an 8-hour TWA of 5 ppm, with no ceiling value, and ACGIH has a TWA of 2 ppm. with a 5 ppm STEL. No doubt, 5 ppm should be one of the alarm set points on a dual alarm unit, and should be the setting on a single-alarm equipped instrument. Depending on individual circumstances, an alarm-1 setting of 2 and alarm-2 setting of 5 seem appropriate.

In conclusion, there is no pat answer to the alarm setting question, as individual applications will vary, and although OSHA standards have the force of law, the other learned institutions may disagree on allowable levels.

With practical experience that is simply unmatched in the industry, Interscan stands ready to help you with all aspects of your toxic gas monitoring applications.

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