Silica law and exposure limits

Every jurisdiction in North America regulates respirable crystalline silica, but the rules differ in their limits, their structure and how much they prescribe. This lesson explains how exposure limits work, which limits and rules apply where you work, and what employers must do once silica is identified. It summarizes the law as it stood in October 2026. Regulations change, so always check the current version and any guidance from your regulator.

How occupational exposure limits work

An occupational exposure limit, or OEL, is the airborne concentration that a worker may be exposed to over a defined period. For silica, limits are expressed as an 8-hour time-weighted average (TWA) concentration of the respirable fraction, in milligrams per cubic metre (mg/m³) or micrograms per cubic metre (µg/m³). One mg/m³ equals 1,000 µg/m³, so 0.025 mg/m³ is the same as 25 µg/m³ and 0.05 mg/m³ is 50 µg/m³.

Three points about OELs are easy to misunderstand:

• An OEL is not a line between safe and unsafe. For a carcinogen like silica there is no known safe level. Limits are set partly on health evidence and partly on what is technically and economically achievable, so many jurisdictions also require exposure to be kept as low as reasonably achievable.

• Limits apply to the respirable fraction. A sample of total dust cannot be compared with a respirable silica limit.

• Limits assume a standard schedule. Most are based on 8 hours a day and 40 hours a week. Longer shifts mean more exposure and less recovery time, so some jurisdictions require the limit to be adjusted.

Who sets the numbers

Two bodies influence silica limits across North America without having legal authority of their own. The American Conference of Governmental Industrial Hygienists (ACGIH) publishes threshold limit values (TLVs). Its TLV for respirable crystalline silica, both quartz and cristobalite, is 0.025 mg/m³ as an 8-hour TWA. Many Canadian jurisdictions adopt ACGIH TLVs into law. The US National Institute for Occupational Safety and Health (NIOSH) publishes recommended exposure limits (RELs); its REL for respirable crystalline silica is 0.05 mg/m³ as a TWA for up to a 10-hour day. RELs are not enforceable but are often used as good practice.

The law in Canada

Occupational health and safety in Canada is mainly provincial and territorial, so the silica rules depend on where you work. Federally regulated workplaces follow the Canada Occupational Health and Safety Regulations (COHSR), which adopt ACGIH TLVs. The table summarizes the main rules. Most provinces have adopted the ACGIH TLV of 0.025 mg/m³; Ontario is the notable exception with a higher limit for quartz.

Why the limits differ

The spread between 25 and 100 µg/m³ is large. It reflects different decisions about the balance between health evidence and feasibility. OSHA's 2016 rule estimated that a 50 µg/m³ PEL would prevent hundreds of deaths and hundreds of new silicosis cases a year in the United States compared with the old limits, while recognizing that lower levels were difficult to measure and achieve reliably with the methods available then. ACGIH sets its TLVs on health grounds alone. Ontario has been urged by unions and health advocates to adopt the ACGIH value. Whatever the legal limit, good practice is to keep exposure as low as reasonably achievable, because risk continues below every current limit.

Mixtures of quartz and cristobalite

Where a material contains both quartz and cristobalite, the exposures are additive because both damage the lungs in the same way. Where the limits differ, as they do in Ontario, the combined exposure is assessed with the mixture formula: each concentration is divided by its own limit and the fractions are added. A total greater than 1 means the combined limit is exceeded. Where the limits are the same, as with the ACGIH TLV, the concentrations can simply be added and compared with the single limit.

def mixture_index(parts):
    """parts: list of (name, concentration mg/m3, limit mg/m3)."""
    total = 0.0
    for name, conc, limit in parts:
        frac = conc / limit
        total += frac
        print(f"  {name:12s} {conc:.3f} mg/m3 against {limit:.3f}: fraction {frac:.2f}")
    return total

sample = [("quartz", 0.040, 0.10), ("cristobalite", 0.020, 0.05)]
print("Limits differ by form (as in Ontario):")
e = mixture_index(sample)
print(f"  mixture index {e:.2f} -> {'over' if e > 1 else 'within'} the combined limit")

print("Single limit for all forms (ACGIH TLV 0.025):")
total = sum(c for _, c, _ in sample)
print(f"  combined {total:.3f} mg/m3 is {total/0.025:.1f} times the limit")

print("Single limit for all forms (OSHA PEL 0.050):")
print(f"  combined {total:.3f} mg/m3 is {total/0.050:.1f} times the PEL")

Limits differ by form (as in Ontario):
  quartz       0.040 mg/m3 against 0.100: fraction 0.40
  cristobalite 0.020 mg/m3 against 0.050: fraction 0.40
  mixture index 0.80 -> within the combined limit
Single limit for all forms (ACGIH TLV 0.025):
  combined 0.060 mg/m3 is 2.4 times the limit
Single limit for all forms (OSHA PEL 0.050):
  combined 0.060 mg/m3 is 1.2 times the PEL

The same sample is within the combined limit under the Ontario approach, well over the ACGIH TLV and above the OSHA PEL. The result depends on the rule that applies, and the safest interpretation is the one most protective of the worker.

Long shifts

Exposure limits based on 8-hour days may not protect workers on 10- or 12-hour shifts, because they breathe dust for longer and have less time to clear it before the next shift. Two approaches are common.

The Brief and Scala model reduces the limit by a factor that accounts for both longer exposure and shorter recovery:

where h is the shift length in hours. The adjusted limit is the 8-hour limit multiplied by RF. For a 12-hour shift, RF = (8 ÷ 12) × (12 ÷ 16) = 0.5, so the limit is halved.

Some Canadian regulators provide their own adjustment methods. Ontario's ministry publishes guidance on adjusting OELs for unusual work schedules, and Alberta's OHS Code includes an adjustment rule, supplemented since July 2023 by a director's approval that allows employers to consider the full weekly schedule. Check which method your regulator accepts.

def brief_scala(limit, hours):
    rf = (8 / hours) * ((24 - hours) / 16)
    return rf, limit * rf

for limit, label_ in ((0.025, "ACGIH TLV"), (0.050, "OSHA PEL")):
    for h in (8, 10, 12):
        rf, adj = brief_scala(limit, h)
        print(f"{label_:9s} {h:2d}-hour shift: RF {rf:.2f}, adjusted limit {adj*1000:.1f} ug/m3")

ACGIH TLV  8-hour shift: RF 1.00, adjusted limit 25.0 ug/m3
ACGIH TLV 10-hour shift: RF 0.70, adjusted limit 17.5 ug/m3
ACGIH TLV 12-hour shift: RF 0.50, adjusted limit 12.5 ug/m3
OSHA PEL   8-hour shift: RF 1.00, adjusted limit 50.0 ug/m3
OSHA PEL  10-hour shift: RF 0.70, adjusted limit 35.0 ug/m3
OSHA PEL  12-hour shift: RF 0.50, adjusted limit 25.0 ug/m3

What employers must do

Although the details vary, every jurisdiction expects employers to work through the same sequence once silica is present:

In BC, the exposure control plan is a specific legal requirement with defined contents, and WorkSafeBC provides templates and an online silica control tool that helps employers select controls for common construction tasks. Ontario's designated substance regulation requires an assessment and, where workers are likely to be exposed, a written control program developed in consultation with the joint health and safety committee or representative. In all provinces, the joint committee or representative should be consulted on the assessment and plan.

Housekeeping rules

Canadian regulators consistently prohibit or discourage dry sweeping and the use of compressed air to clean silica dust from surfaces and clothing, and require wet methods or vacuums with high-efficiency filters instead. In BC, the requirement is written into the regulation for silica-containing dust.