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 meter (mg/m³) or micrograms per cubic meter (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 the United States

In the United States, OSHA's 2016 respirable crystalline silica rule created two standards: 29 CFR 1926.1153 for construction and 29 CFR 1910.1053 for general industry and maritime. Both set a permissible exposure limit (PEL) of 50 µg/m³ and an action level of 25 µg/m³, each as an 8-hour TWA. State plans must be at least as effective as federal OSHA, and some, notably California, go further. Mining is regulated separately by MSHA.

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

OSHA's silica PEL applies to the total of all forms of respirable crystalline silica together: quartz, cristobalite and tridymite are added and compared with 50 µg/m³. Where a jurisdiction or an internal standard sets different limits for different forms, the mixture formula is used instead: each concentration is divided by its own limit and the fractions are added, with a total over 1 meaning the combined limit is exceeded.

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.

OSHA's silica standards express the PEL as an 8-hour TWA and do not prescribe an adjustment for longer shifts, although exposure must be assessed over the whole shift. Many hygienists apply the Brief and Scala model as good practice. OSHA's lead standard, by contrast, does include a formula, as Lesson 8 explains.

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:

Both OSHA standards require a written exposure control plan describing the tasks that involve silica, the engineering controls, work practices and respirators used for each, the housekeeping measures and, in construction, procedures to restrict access to areas where exposures may be high. Construction employers must designate a competent person to implement the plan. Lesson 4 explains the choice between Table 1 and exposure assessment.

Housekeeping rules

Both OSHA standards prohibit dry sweeping and dry brushing where they could contribute to exposure unless wet sweeping, HEPA vacuuming or other methods are not feasible, and prohibit compressed air for cleaning clothing or surfaces unless it is used with a ventilation system that captures the dust or no feasible alternative exists.