Exposure limits and the law

Hydrogen sulfide is regulated in two overlapping ways. Occupational health and safety law sets exposure limits and requires employers to assess hazards, control exposures, train workers and plan for emergencies. Energy and environmental regulators add rules for sour wells, pipelines and facilities, including emergency planning around them. This lesson describes the main limits and laws.

Kinds of exposure limits

Exposure limits for gases come in several forms. A time-weighted average (TWA) limit applies to the average concentration over a work shift, usually eight hours. A short-term exposure limit (STEL) applies to a 15-minute average that must not be exceeded at any time during the shift. A ceiling limit must not be exceeded at any instant. For hydrogen sulfide, short-term and ceiling limits matter most, because harm depends mainly on peak concentrations.

The ACGIH, a non-governmental scientific body in the United States, publishes threshold limit values (TLVs). Its TLVs for hydrogen sulfide, adopted in 2010, are 1 ppm as an eight-hour TWA and 5 ppm as a STEL. Many jurisdictions adopt ACGIH values into their law, sometimes years later and sometimes with changes. Others keep older limits.

Limits in Canada

Each province and territory sets its own exposure limits, and federally regulated workplaces such as interprovincial pipelines, ports and offshore installations follow the Canada Occupational Health and Safety Regulations under the Canada Labour Code. Some jurisdictions adopt the ACGIH TLVs by reference, so the 1 ppm TWA and 5 ppm STEL apply there. Others still use the older values of about 10 ppm. As examples, Alberta's OHS Code sets an eight-hour limit of 10 ppm with a 15 ppm ceiling, British Columbia sets a ceiling of 10 ppm, and Saskatchewan sets 10 ppm as an eight-hour average with a 15 ppm 15-minute limit. Ontario's Regulation 833 table should be checked directly for the current value. Because the values differ, a company working in several provinces often adopts the most protective limit everywhere.

Calculating exposure

To compare an exposure with a TWA limit, average the concentrations over the shift, weighted by time. Time with no exposure counts as zero. For short-term limits, look at the highest 15-minute average. For ceilings, look at the highest instantaneous reading.

# A wastewater operator's shift (minutes, ppm) from a datalogging personal monitor.
periods = [(120, 0.2), (45, 4.0), (15, 9.0), (60, 0.5), (90, 2.5), (150, 0.3)]
total_min = sum(m for m, _ in periods)
twa_8h = sum(m * c for m, c in periods) / 480        # unexposed time beyond the log counts as 0
peak_15 = max(c for m, c in periods if m >= 15)
print(f"logged minutes: {total_min}")
print(f"8-hour TWA: {twa_8h:.2f} ppm")
print(f"highest 15-minute average: {peak_15:.1f} ppm")
for name, twa, stel in [("ACGIH TLV", 1, 5), ("Alberta OEL", 10, 15)]:
    print(f"{name:12s}: TWA {'over' if twa_8h > twa else 'under'} {twa} ppm; "
          f"short-term {'over' if peak_15 > stel else 'under'} {stel} ppm")

logged minutes: 480
8-hour TWA: 1.33 ppm
highest 15-minute average: 9.0 ppm
ACGIH TLV   : TWA over 1 ppm; short-term over 5 ppm
Alberta OEL : TWA under 10 ppm; short-term under 15 ppm

The same exposure can be acceptable under one limit and well over another. Under the ACGIH values, this operator exceeded both the eight-hour TLV and the short-term limit; under the older 10 ppm and 15 ppm values, the exposure would comply. The employer should apply the limit that legally applies and should consider the more protective one, especially since the ACGIH values reflect more recent evidence.

General duties

Exposure limits are only part of the law. Every Canadian OHS statute imposes a general duty on employers to take every precaution reasonable in the circumstances to protect workers. Regulations in every jurisdiction require hazard assessment, control of hazardous substances, confined space programs, WHMIS 2015 labelling, safety data sheets and training, and emergency procedures. The Alberta OHS Code, for example, requires a written code of practice for hydrogen sulfide where workers may be exposed, and its oil and gas well rules require competent supervision for hydrogen sulfide, breathing apparatus for emergency response and testing during drill stem tests. Confined space rules, covered in HSE-211, apply whenever hydrogen sulfide is in or may enter a confined space.

Energy regulators in Canada

Provincial energy regulators add rules for sour oil and gas. The Alberta Energy Regulator's Directive 071 requires licensees to prepare emergency response plans for sour wells, pipelines and facilities and to calculate an emergency planning zone (EPZ) around them: an area where residents and the public must be identified, consulted and included in emergency procedures. EPZs are calculated with a dispersion model. In November 2025 the AER released a new model, AERH2S, replacing the older ERCBH2S tool; it became mandatory for new applications on 1 January 2026 and uses a reasonable worst-case approach that accounts for safety systems such as emergency shutdown valves. The BC Energy Regulator and Saskatchewan's energy ministry have comparable requirements for emergency planning around sour operations. Gas processing plants that remove hydrogen sulfide and recover sulfur are also regulated for sulfur emissions and flaring.

Emergency planning distances

Regulators use dispersion models to estimate how far a release could travel at dangerous concentrations. The models consider the release rate, the hydrogen sulfide content, the weather and the terrain. The Texas formula is a simple example of such a calculation, using the hydrogen sulfide mole fraction and the maximum escape volume per day. It is shown here to illustrate how distances grow with release size, not as a planning tool for any jurisdiction.

# Texas Rule 36 radius-of-exposure formulas (feet), for illustration only.
# x100 = (1.589 * mole_fraction * Q) ** 0.6258 ; x500 = (0.4546 * mole_fraction * Q) ** 0.6258
# Q = maximum escape volume, cubic feet per day at standard conditions.
def roe(mole_fraction, q_cfd):
    x100 = (1.589 * mole_fraction * q_cfd) ** 0.6258
    x500 = (0.4546 * mole_fraction * q_cfd) ** 0.6258
    return x100, x500

for frac, q in [(0.001, 1_000_000), (0.01, 1_000_000), (0.05, 5_000_000)]:
    a, b = roe(frac, q)
    print(f"H2S {frac*100:4.1f} %, Q {q:>9,} cf/d: 100 ppm ROE {a:7,.0f} ft ({a*0.3048:6,.0f} m); "
          f"500 ppm ROE {b:6,.0f} ft ({b*0.3048:5,.0f} m)")

H2S  0.1 %, Q 1,000,000 cf/d: 100 ppm ROE     101 ft (    31 m); 500 ppm ROE     46 ft (   14 m)
H2S  1.0 %, Q 1,000,000 cf/d: 100 ppm ROE     426 ft (   130 m); 500 ppm ROE    195 ft (   59 m)
H2S  5.0 %, Q 5,000,000 cf/d: 100 ppm ROE   3,191 ft (   973 m); 500 ppm ROE  1,458 ft (  444 m)

Even a modest well with 1 per cent hydrogen sulfide and a million cubic feet per day of potential escape gives a 100 ppm radius of hundreds of metres. That is why sour sites have signs, controlled access, wind socks and emergency plans that reach beyond the lease boundary.

Consensus standards and training rules

Beyond legislation, industry standards shape practice. Energy Safety Canada publishes industry recommended practices and administers H2S Alive, the most widely required hydrogen sulfide training in the Canadian energy sector, with an eight-hour course and a three-year certificate. CSA standards cover gas detectors and respiratory protection, and the WHMIS 2015 classification of hydrogen sulfide (flammable gas, gas under pressure, acute toxicity by inhalation) appears on cylinder labels and safety data sheets.

Keeping up with changing rules

Exposure limits and energy regulations change. Provinces review their occupational exposure limits periodically, often moving toward the current ACGIH values, and energy regulators update directives and models, as the AER did with AERH2S in 2025 and 2026. A hydrogen sulfide program should name a person responsible for tracking changes, check the regulations at least once a year, and update alarm set points, procedures and training when limits change. Contractors working across provinces should keep a simple table of the limits in each jurisdiction where they work and record which value their program uses.

It also helps to understand why limits differ. Legal limits reflect the evidence available when they were adopted, the regulator's process and sometimes feasibility. Recommendations such as the ACGIH TLVs are based on health evidence alone. When a legal limit is higher than a current recommendation, the legal limit is the minimum the law requires, not proof that exposures up to it are harmless. Employers who choose the lower value protect workers better and are rarely caught out when the law later catches up.

Finally, remember that limits apply to the air workers breathe. A fixed detector reading in a building is useful, but compliance with a TWA or STEL is judged from the worker's breathing zone, ideally from personal monitoring or datalogs, as the next lessons explain.