Oil and gas: the H₂S and benzene limits people get wrong

Two of the exposure limits that matter most in this industry are the ones people get wrong with the most confidence. Both are in OSHA's Table Z-2, and neither works like a simple 8-hour time-weighted average (TWA).

Hydrogen sulfide: general industry has no 8-hour limit

OSHA has separate standards for construction, for shipyards and for general industry. For general industry, Table Z-2 gives hydrogen sulfide (H₂S) no permissible exposure limit (PEL) as an 8-hour TWA. What it sets instead is:

• Acceptable ceiling concentration: 20 parts per million (ppm)
• Acceptable maximum peak above that ceiling: 50 ppm for 10 minutes, once only in an 8-hour shift, and only if no other measurable exposure occurs

That last condition deserves a second reading. A site with steady low-level hydrogen sulfide plus a tank gauging peak each day does not get to use the 50 ppm allowance at all.

One pairing circulates widely in this industry: a 10 ppm TWA with a 15 ppm short-term exposure limit (STEL). That pair is not federal law for general industry. The 10 ppm 8-hour TWA is real, but it belongs to OSHA's construction and shipyard standards, and the 15 ppm STEL is a California value from Cal/OSHA. Federal OSHA has no STEL for hydrogen sulfide.

As a result, a workover crew operating under the construction standards can be subject to one limit while the production operators next to them are subject to another, on the same pad on the same day. That is no small detail for the person writing the job safety analysis.

For comparison, these are the health-based limits:

• NIOSH has a recommended exposure limit (REL) of 10 ppm as a ceiling limit over 10 minutes.
• ACGIH has a threshold limit value (TLV) of 1 ppm as a TWA, with a 5 ppm STEL.
• The level that is immediately dangerous to life or health is 100 ppm.

Because the limit is a ceiling and a peak, a full-shift pump sample is the wrong tool. The survey has to target the moments that matter:

• Tank gauging
• Sample collection
• Pigging
• Separator work
• Produced water handling
• Vessel entry

→ Hydrogen sulfide air monitoring

Benzene: which standard applies is not obvious

OSHA's benzene standard sets a 1 ppm TWA, a 5 ppm STEL over 15 minutes and a 0.5 ppm action level. The standard backs those limits with exposure monitoring, medical surveillance and regulated areas.

But the benzene standard exempts a list of operations, and that list includes oil and gas drilling, production and servicing. For exempt work, the limit falls back to Table Z-2:

• 10 ppm 8-hour TWA
• 25 ppm acceptable ceiling concentration
• 50 ppm acceptable maximum peak, for 10 minutes

That is a tenfold difference in the TWA limit, determined by which side of the exemption your crew is on, and it is easier to fall outside the exemption than people assume. Work at a terminal, a gas plant, a treating plant or a transfer operation may not count as "drilling, production or servicing" work. A company that does both kinds of work should not run a single benzene policy.

The health-based limits are where the exempt limit becomes hardest to defend.

• ACGIH's TLV is 0.02 ppm as a TWA, with a 0.1 ppm STEL. ACGIH adds a skin notation, meaning benzene can be absorbed through the skin, and classifies benzene as A1, a confirmed human carcinogen.
• NIOSH's REL is 0.1 ppm.

Table Z-2's 10 ppm is 500 times the ACGIH value, and benzene is a confirmed cause of leukemia.

→ Benzene air monitoring · OSHA PELs and ACGIH TLVs compared

Tank gauging is the task that kills people

Manually gauging and sampling a tank through an open thief hatch releases a concentrated burst of hydrocarbon vapor directly into the breathing zone of the person standing over the hatch. NIOSH and OSHA issued a joint hazard alert on this, after a cluster of deaths among workers found collapsed at open hatches.

The cause is not only oxygen displacement, and not only hydrogen sulfide. At high concentrations, hydrocarbon vapor is a fast-acting poison that can make the heart overreact to the body's own stress hormones (catecholamines). That is why a worker can collapse with no warning and no time to get clear. A four-gas monitor set for the lower explosive limit (LEL) will not give a useful alarm for that hazard.

Respirable crystalline silica from sand handling

Sand handling generates respirable crystalline silica. OSHA's general industry silica standard applies, and that standard has no Table 1. Table 1 is the shortcut that specifies dust controls for listed tasks, and it belongs to the construction silica standard. The general industry standard has nothing like it. What you get instead is the performance option, which requires data.

Hydraulic fracturing was given a phased compliance schedule. Most obligations took effect in June 2018, and engineering controls for silica in fracturing work were due by June 23, 2021. That deadline has passed, and the phase-in no longer works as an explanation.

The dust comes from sand movers, belt transfers, blenders, the dragon tail and the thief hatches on sand equipment. Wind matters a great deal, and the duration and frequency of each task matter even more.

→ Respirable crystalline silica air monitoring for general industry

Diesel exhaust, noise and the two-standard problem

Diesel particulate matter has no OSHA PEL, because OSHA does not regulate it as a named substance at all. That does not make a laboratory result for elemental carbon useless. It makes the result a health-based number, not a pass or fail against a legal limit.

Noise is split between two standards the same way hydrogen sulfide is. OSHA's general industry noise standard requires audiometric testing, training and a full hearing conservation program at or above an action level of 85 decibels (dBA). The construction noise standard sets a 90 decibel PEL, with no matching audiometric testing requirement. The same crew on the same compressor pad can face two different sets of requirements, depending on how the work is classified.

One note on process safety: OSHA's Process Safety Management standard exempts oil and gas well drilling and servicing operations, and remote sites that are normally unoccupied are exempt separately. That exemption does not follow you into a gas plant. Process safety work is outside the four fields we work in.

→ Noise surveys