OSHA Safety and Health Regulations for Construction (29 CFR Part 1926) 1926.55: 1926.55 Gases, vapors, fumes, dusts, and mists

Employee exposure to each airborne substance in Table 1 (permissible exposure limits for airborne contaminants) or Table 2 (mineral dusts) must be kept within the listed limits, determined from breathing-zone air samples. A limit marked (C) is a ceiling that may never be exceeded; where instantaneous monitoring is not feasible, the ceiling is judged as a 15-minute TWA that may not be exceeded at any point in the workday. Every other limit is applied as an 8-hour TWA for each shift. To comply, the employer first puts in place administrative or engineering controls wherever feasible. Where these fall short of full compliance, protective equipment or other measures keep exposure within the limits; each such device or technical measure needs prior approval for that specific use from a competent industrial hygienist or another person with the technical qualifications, and respirator use must comply with 1926.103. The section does not govern asbestos, tremolite, anthophyllite or actinolite dust (1926.1101 applies) or formaldehyde (1910.1048 applies). Table 1 lists several hundred substances with ppm and mg/m3 values and a skin designation (X marks a dermal hazard); for example carbon monoxide 50 ppm, hydrogen sulfide 10 ppm, toluene 200 ppm, acetone 1000 ppm, ammonia 50 ppm, Portland cement 15 mg/m3 total dust, and particulates not otherwise regulated 15 mg/m3 total dust. Many entries cross-refer to substance-specific standards (lead 1926.62, benzene 1926.1128, cadmium 1926.1127, chromium VI 1926.1126, beryllium 1926.1124, respirable crystalline silica 1926.1153, the carcinogen sections, vinyl chloride, acrylonitrile, ethylene oxide, methylene chloride and 1,3-butadiene). Where CAS numbers appear they are informational only; enforcement follows the substance name. Fallback limits apply where a substance-specific standard is stayed or not in effect: the Table 2 silica formula for quartz and other crystalline silica where 1926.1153 is not in effect, a beryllium limit of 0.002 mg/m3 where 1926.1124 is not in effect, a chromium VI limit of 0.1 mg/m3 (tert-butyl chromate as a ceiling, other compounds as CrO3 8-hour TWA), and 10 ppm TWA benzene for sectors excluded from 1926.1128. Table 2 sets mineral dust limits in millions of particles per cubic foot (mppcf, impinger samples counted by light-field technique): crystalline quartz 250 divided by (percent SiO2 plus 5), amorphous silica including diatomaceous earth 20, mica, soapstone and non-asbestiform talc 20, Portland cement 50, natural graphite 15, fibrous talc at the asbestos limit, and inert or nuisance dust 50 mppcf or 15 mg/m3, whichever is smaller. Simple asphyxiants (for example acetylene, argon, methane, nitrogen, propane) are marked E: the limiting factor is available oxygen, which must be at least 19.5% and within the Part 1926 explosion provisions. Gasoline and petroleum distillates (A3) require their benzene, other aromatic and additive content to be established in order to set the appropriate limit.