Mineral dust exposure limits

The PEL for respirable crystalline silica (quartz) in Table Z-3 is calculated using the mass formula 10/(% SiO2 + 2) mg/m3. Use the percent crystalline silica measured from airborne samples in the denominator and apply the formula in Table Z-3 - Mineral Dusts.

• The table itself and its footnotes explain the formula and that the percentage of crystalline silica is normally determined from airborne samples (see note b in the table).
• When respirable crystalline silica is regulated by the separate respirable crystalline silica standard, employers must follow 29 CFR 1910.1053 instead of Table Z-3 if that standard is in effect.

You set the PEL for cristobalite and tridymite at one half of the value calculated by the quartz formula in Table Z-3. Table Z-3 explicitly says to "Use ½ the value calculated from the count or mass formulae for quartz" for cristobalite and tridymite, so calculate the quartz limit (for example using 10/(% SiO2 + 2) mg/m3) and divide that result by 2, as shown in Table Z-3 - Mineral Dusts.

The percent SiO2 used in the Table Z-3 silica formula should be determined from airborne samples unless another validated method is applicable. Table Z-3 note b says "The percentage of crystalline silica in the formula is the amount determined from airborne samples, except in those instances in which other methods have been shown to be applicable," so you should base the percent on representative airborne sampling as described in Table Z-3 - Mineral Dusts.

You determine the respirable fraction using a size selector with the selector characteristics shown in Table Z-3 (for example, 90% passing at aerodynamic diameter 2 μm, 75% at 2.5 μm, 50% at 3.5 μm, 25% at 5 μm, 0% at 10 μm). Table Z-3 note e explains these percent-passing values and states the measurements refer to use of an AEC (now NRC) instrument; see Table Z-3 - Mineral Dusts.

• For coal dust the table notes the respirable fraction is determined with an MRE instrument and provides the corresponding values (see the same note).

Table Z-3's silica entries apply only when the respirable crystalline silica standard at 29 CFR 1910.1053 is stayed or otherwise not in effect. Footnote f in Table Z-3 - Mineral Dusts states: "This standard applies to any operations or sectors for which the respirable crystalline silica standard, 1910.1053, is stayed or is otherwise not in effect."

• If 1910.1053 is in effect for your operations, you must comply with that silica-specific standard rather than the Table Z-3 formula.

The exposure limit for amorphous (including natural diatomaceous earth) in Table Z-3 is 80 mg/m3 (mass) or 20 mppcf (count). Table Z-3 lists "Amorphous, including natural diatomaceous earth" with mppcf = 20 and mg/m3 = 80, as shown in Table Z-3 - Mineral Dusts.

For talc that does not contain asbestos, Table Z-3 lists a limit of 20 mppcf (and no mass mg/m3 value is given); for talc that contains asbestos you must use the asbestos limit. Table Z-3 states "Talc (not containing asbestos) 20c" and "Talc (containing asbestos) Use asbestos limit"—when talc contains asbestos you should follow the asbestos standard at 29 CFR 1910.1001.

• Note also the footnote c: if talc contains less than 1% quartz the listed talc entry applies; if talc has 1% or more quartz, use the quartz calculation in Table Z-3.

Table Z-3 sets the PNOR (inert or nuisance dust) limits at 5 mg/m3 for the respirable fraction and 15 mg/m3 for total dust. The table labels these under "Inert or Nuisance Dust" and notes that this is the same as the Particulates Not Otherwise Regulated (PNOR) limit in Table Z-1; see Table Z-3 - Mineral Dusts.

For coal dust in Table Z-3, if the respirable fraction contains less than 5% crystalline silica the respirable PEL is 2.4 mg/m3 (with an associated total-dust value of 10 mg/m3); if the respirable fraction contains greater than 5% crystalline silica you must use the silica formula (% SiO2 + 2) to calculate the respirable limit. See the coal dust rows and note e in Table Z-3 - Mineral Dusts.

• The table also notes that when using the MRE sampler for coal dust the number corresponding to 2.4 mg/m3 is 4.5 mg/m3 (see note e).

Convert mppcf (million particles per cubic foot) to particles per cubic centimeter by multiplying the mppcf value by 35.3 (mppcf × 35.3 = particles per cc). Table Z-3's note gives that conversion: "mppcf × 35.3 = million particles per cubic meter = particles per c.c."—see Table Z-3 - Mineral Dusts.

The size selector must match the percent-passing characteristics listed in Table Z-3 note e (e.g., 90% passing at 2 μm, 75% at 2.5 μm, 50% at 3.5 μm, 25% at 5 μm, 0% at 10 μm) and the table indicates these measurements refer to use of an AEC (now NRC) instrument. Follow the selector characteristics in Table Z-3 - Mineral Dusts to determine the respirable fraction for applying the silica limits.

If a silicate product (for example certain talc or other silicates) contains 1% or more crystalline silica you must use the quartz (respirable silica) limit rather than the silicate entry. Table Z-3 note c states that entries like talc are for materials "containing less than 1% quartz; if 1% quartz or more, use quartz limit," as shown in Table Z-3 - Mineral Dusts.

If airborne respirable silica measures 40% SiO2, apply the Table Z-3 mass formula 10/(% SiO2 + 2): 10 / (40 + 2) = 10 / 42 = 0.238 mg/m3. Use that mg/m3 value as the PEL for respirable quartz under Table Z-3 - Mineral Dusts.

• Remember the percent SiO2 should be determined from representative airborne samples per note b in the table.

When talc contains asbestos you must follow the asbestos standard, 29 CFR 1910.1001, rather than the talc entry in Table Z-3. Table Z-3 explicitly says "Talc (containing asbestos) Use asbestos limit," and OSHA's asbestos standard contains the specific exposure controls and requirements for asbestos-containing materials.

• For practical guidance on whether a remediation activity is covered by the construction asbestos standard or the general‑industry asbestos standard, see OSHA's letter of interpretation on asbestos remediation protocols at https://www.osha.gov/laws-regs/standardinterpretations/2024-11-14.

Table Z-3 refers to AEC (now NRC) instruments for determining the respirable fraction for most mineral dusts and to an MRE instrument specifically for coal dust. Note e in Table Z-3 - Mineral Dusts explains that the measurements refer to the use of an AEC (now NRC) instrument, while the respirable fraction of coal dust is determined with an MRE and the corresponding numeric value is provided.

Yes — Table Z-3 notes that all inert or nuisance dusts not listed specifically are covered by the same limits as the Particulates Not Otherwise Regulated (PNOR) limit in Table Z-1. Table Z-3 note d explicitly states that the inert/nuisance dust limit is the same as the PNOR limit in Table Z-1; see Table Z-3 - Mineral Dusts.

When 29 CFR 1910.1053 (the respirable crystalline silica standard) is in effect for your operation you must comply with 1910.1053 rather than relying on the Table Z-3 silica formula. Footnote f of Table Z-3 - Mineral Dusts states that Table Z-3 applies only for sectors or operations for which the respirable crystalline silica standard is stayed or otherwise not in effect.

If you measure respirable coal dust with an MRE sampler the value corresponding to the 2.4 mg/m3 AEC-based figure in Table Z-3 is 4.5 mg/m3. Table Z-3 note e explains that the respirable fraction of coal dust is determined with an MRE and that the figure corresponding to 2.4 mg/m3 for coal dust is 4.5 mg/m3; see Table Z-3 - Mineral Dusts.

Table Z-3 itself is a general-industry table of permissible exposure limits and does not change which OSHA standard applies to specific activities; if asbestos remediation involves construction-type activities, OSHA's construction asbestos standard, 29 CFR 1926.1101 (see the interpretation on asbestos remediation protocols for guidance), may apply instead of the general-industry asbestos rule at 29 CFR 1910.1001.

• For practical guidance on whether remediation work should be covered by the construction or general‑industry asbestos standard, see OSHA's letter of interpretation at https://www.osha.gov/laws-regs/standardinterpretations/2024-11-14.

Those two formula rows in Table Z-3 are the count and mass formulae used to calculate allowable exposures: the mppcf (count) formula is 250/(% SiO2 + 5) and the mg/m3 (mass) formula is 10/(% SiO2 + 2). Table Z-3 presents both the particle-count formula (in mppcf) and the mass-concentration formula (in mg/m3); see Table Z-3 - Mineral Dusts.