###### Editorial Note on Part 1926

#### Editorial Notes:

1. At 44 FR 8577, Feb. 9, 1979, and corrected at 44 FR 20940, Apr. 6, 1979, OSHA reprinted without change the entire text of 29 CFR part 1926 together with certain General Industry Occupational Safety and Health Standards contained in 29 CFR part 1910, which have been identified as also applicable to construction work. This republication developed a single set of OSHA regulations for both labor and management forces within the construction industry.

2. Nomenclature changes to part 1926 appear at 84 FR 21597, May 14, 2019.

#### § 1926.55 Gases, vapors, fumes, dusts, and mists.

(a) Employers must limit an employee's exposure to any substance listed in Table 1 or 2 of this section in accordance with the following:

(1) *Substances with limits preceded by (C)—Ceiling Values.* An employee's exposure, as determined from breathing-zone air samples, to any substance in Table 1 of this section with a permissible exposure limit preceded by (C) must at no time exceed the exposure limit specified for that substance. If instantaneous monitoring is not feasible, then the employer must assess the ceiling as a 15-minute time-weighted average exposure that the employer cannot exceed at any time during the working day.

(2) *Other substances—8-hour Time Weighted Averages.* An employee's exposure, as determined from breathing-zone air samples, to any substance in Table 1 or 2 of this section with a permissible exposure limit not preceded by (C) must not exceed the limit specified for that substance measured as an 8-hour time-weighted average in any work shift.

(b) To achieve compliance with paragraph (a) of this section, administrative or engineering controls must first be implemented whenever feasible. When such controls are not feasible to achieve full compliance, protective equipment or other protective measures shall be used to keep the exposure of employees to air contaminants within the limits prescribed in this section. Any equipment and technical measures used for this purpose must first be approved for each particular use by a competent industrial hygienist or other technically qualified person. Whenever respirators are used, their use shall comply with § 1926.103.

(c) Paragraphs (a) and (b) of this section do not apply to the exposure of employees to airborne asbestos, tremolite, anthophyllite, or actinolite dust. Whenever any employee is exposed to airborne asbestos, tremolite, anthophyllite, or actinolite dust, the requirements of § 1926.1101 shall apply.

(d) Paragraphs (a) and (b) of this section do not apply to the exposure of employees to formaldehyde. Whenever any employee is exposed to formaldehyde, the requirements of § 1910.1048 of this title shall apply.

| Substance | CAS No.d | ppma | mg/m3 b | Skindesignation * |
| --- | --- | --- | --- | --- |
| Abate; see Temephos | | | | |
| Acetaldehyde | 75-07-0 | 200 | 360 | — |
| Acetic acid | 64-19-7 | 10 | 25 | — |
| Acetic anhydride | 108-24-7 | 5 | 20 | — |
| Acetone | 67-64-1 | 1000 | 2400 | — |
| Acetonitrile | 75-05-8 | 40 | 70 | — |
| 2-Acetylaminofluorine; see § 1926.1114 | 53-96-3 | | | |
| Acetylene | 74-86-2 | E | | |
| Acetylene dichloride; see 1,2-Dichloroethylene | | | | |
| Acetylene tetrabromide | 79-27-6 | 1 | 14 | — |
| Acrolein | 107-02-8 | 0.1 | 0.25 | — |
| Acrylamide | 79-06-1 | — | 0.3 | X |
| Acrylonitrile; see § 1926.1145 | 107-13-1 | | | |
| Aldrin | 309-00-2 | — | 0.25 | X |
| Allyl alcohol | 107-18-6 | 2 | 5 | X |
| Allyl chloride | 107-05-1 | 1 | 3 | — |
| Allyl glycidyl ether (AGE) | 106-92-3 | (C)10 | (C)45 | — |
| Allyl propyl disulfide | 2179-59-1 | 2 | 12 | — |
| alpha-Alumina | 1344-28-1 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Alundum; see alpha-Alumina | | | | |
| 4-Aminodiphenyl; see § 1926.1111 | 92-67-1 | | | |
| 2-Aminoethanol; see Ethanolamine | | | | |
| 2-Aminopyridine | 504-29-0 | 0.5 | 2 | — |
| Ammonia | 7664-41-7 | 50 | 35 | — |
| Ammonium sulfamate | 7773-06-0 | | | |
| Total dust | | — | 15 | — |
| Respirable fraction | | — | 5 | — |
| n-Amyl acetate | 628-63-7 | 100 | 525 | — |
| sec-Amyl acetate | 626-38-0 | 125 | 650 | — |
| Aniline and homologs | 62-53-3 | 5 | 19 | X |
| Anisidine (o-, p-isomers) | 29191-52-4 | — | 0.5 | X |
| Antimony and compounds (as Sb) | 7440-36-0 | — | 0.5 | — |
| ANTU (alpha Naphthylthiourea) | 86-88-4 | — | 0.3 | — |
| Argon | 7440-37-1 | E | | |
| Arsenic, inorganic compounds (as As); see § 1926.1118 | 7440-38-2 | — | — | — |
| Arsenic, organic compounds (as As) | 7440-38-2 | — | 0.5 | — |
| Arsine | 7784-42-1 | 0.05 | 0.2 | — |
| Asbestos; see § 1926.1101 | | | | |
| Azinphos-methyl | 86-50-0 | — | 0.2 | X |
| Barium, soluble compounds (as Ba) | 7440-39-3 | — | 0.5 | — |
| Benzeneg; see § 1926.1128 | 71-43-2 | | | |
| Benzidine; see § 1926.1110 | 92-87-5 | | | |
| p-Benzoquinone; see Quinone | | | | |
| Benzo(a)pyrene; see Coal tar pitch volatiles | | | | |
| Benzoyl peroxide | 94-36-0 | — | 5 | — |
| Benzyl chloride | 100-44-7 | 1 | 5 | — |
| Beryllium and beryllium compounds (as Be); see 1926.1124(q) | 7440-41-7 | — | 0.002 | — |
| Biphenyl; see Diphenyl | | | | |
| Bisphenol A; see Diglycidyl ether | | | | |
| Boron oxide | 1303-86-2 | | | |
| Total dust | | — | 15 | — |
| Boron tribromide | 10294-33-4 | 1 | 10 | — |
| Boron trifluoride | 7637-07-2 | (C)1 | (C)3 | — |
| Bromine | 7726-95-6 | 0.1 | 0.7 | — |
| Bromine pentafluoride | 7789-30-2 | 0.1 | 0.7 | — |
| Bromoform | 75-25-2 | 0.5 | 5 | X |
| Butadiene (1,3-Butadiene); see 29 CFR 1910.1051; 29 CFR 1910.19(l) | 106-99-0 | STEL 1 ppm/5 ppm | | — |
| Butanethiol; see Butyl mercaptan | | | | |
| 2-Butanone (Methyl ethyl ketone) | 78-93-3 | 200 | 590 | — |
| 2-Butoxyethanol | 111-76-2 | 50 | 240 | X |
| n-Butyl-acetate | 123-86-4 | 150 | 710 | — |
| sec-Butyl acetate | 105-46-4 | 200 | 950 | — |
| tert-Butyl acetate | 540-88-5 | 200 | 950 | — |
| n-Butyl alcohol | 71-36-3 | 100 | 300 | — |
| sec-Butyl alcohol | 78-92-2 | 150 | 450 | — |
| tert-Butyl alcohol | 75-65-0 | 100 | 300 | — |
| Butylamine | 109-73-9 | (C)5 | (C)15 | X |
| tert-Butyl chromate (as CrO3); see 1926.1126n | 1189-85-1 | | | |
| n-Butyl glycidyl ether (BGE) | 2426-08-6 | 50 | 270 | — |
| Butyl mercaptan | 109-79-5 | 0.5 | 1.5 | — |
| p-tert-Butyltoluene | 98-51-1 | 10 | 60 | — |
| Cadmium (as Cd); see 1926.1127 | 7440-43-9 | | | |
| Calcium carbonate | 1317-65-3 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Calcium oxide | 1305-78-8 | — | 5 | — |
| Calcium sulfate | 7778-18-9 | | | |
| Total dust | | — | 15 | — |
| Respirable fraction | | — | 5 | — |
| Camphor, synthetic | 76-22-2 | — | 2 | — |
| Carbaryl (Sevin) | 63-25-2 | — | 5 | — |
| Carbon black | 1333-86-4 | — | 3.5 | — |
| Carbon dioxide | 124-38-9 | 5000 | 9000 | — |
| Carbon disulfide | 75-15-0 | 20 | 60 | X |
| Carbon monoxide | 630-08-0 | 50 | 55 | — |
| Carbon tetrachloride | 56-23-5 | 10 | 65 | X |
| Cellulose | 9004-34-6 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Chlordane | 57-74-9 | — | 0.5 | X |
| Chlorinated camphene | 8001-35-2 | — | 0.5 | X |
| Chlorinated diphenyl oxide | 55720-99-5 | — | 0.5 | — |
| Chlorine | 7782-50-5 | 1 | 3 | — |
| Chlorine dioxide | 10049-04-4 | 0.1 | 0.3 | |
| Chlorine trifluoride | 7790-91-2 | (C)0.1 | (C)0.4 | — |
| Chloroacetaldehyde | 107-20-0 | (C)1 | (C)3 | — |
| a-Chloroacetophenone (Phenacyl chloride) | 532-27-4 | 0.05 | 0.3 | — |
| Chlorobenzene | 108-90-7 | 75 | 350 | — |
| o-Chlorobenzylidene malononitrile | 2698-41-1 | 0.05 | 0.4 | — |
| Chlorobromomethane | 74-97-5 | 200 | 1050 | — |
| 2-Chloro-1,3-butadiene; see beta-Chloroprene | | | | |
| Chlorodiphenyl (42% Chlorine) (PCB) | 53469-21-9 | — | 1 | X |
| Chlorodiphenyl (54% Chlorine) (PCB) | 11097-69-1 | — | 0.5 | X |
| 1-Chloro,2,3-epoxypropane; see Epichlorohydrin | | | | |
| 2-Chloroethanol; see Ethylene chlorohydrin | | | | |
| Chloroethylene; see Vinyl chloride | | | | |
| Chloroform (Trichloromethane) | 67-66-3 | (C)50 | (C)240 | — |
| bis(Chloromethyl) ether; see § 1926.1108 | 542-88-1 | | | |
| Chloromethyl methyl ether; see § 1926.1106 | 107-30-2 | | | |
| 1-Chloro-1-nitropropane | 600-25-9 | 20 | 100 | — |
| Chloropicrin | 76-06-2 | 0.1 | 0.7 | — |
| beta-Chloroprene | 126-99-8 | 25 | 90 | X |
| Chromium (II) compounds | | | | |
| (as Cr) | 7440-47-3 | — | 0.5 | — |
| Chromium (III) compounds | | | | |
| (as Cr) | 7440-47-3 | — | 0.5 | — |
| Chromium (VI) compounds; See 1926.1126o | | | | |
| Chromium metal and insol. salts (as Cr) | 7440-47-3 | — | 1 | — |
| Chrysene; see Coal tar pitch volatiles | | | | |
| Coal tar pitch volatiles (benzene soluble fraction), anthracene, BaP, phenanthrene, acridine, chrysene, pyrene | 65996-93-2 | — | 0.2 | — |
| Cobalt metal, dust, and fume (as Co) | 7440-48-4 | — | 0.1 | — |
| Copper | 7440-50-8 | | | |
| Fume (as Cu) | | — | 0.1 | — |
| Dusts and mists (as Cu) | | — | 1 | — |
| Corundum; see Emery | | | | |
| Cotton dust (raw) | | — | 1 | |
| Crag herbicide (Sesone) | 136-78-7 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Cresol, all isomers | 1319-77-3 | 5 | 22 | X |
| Crotonaldehyde | 123-73-9; | 2 | 6 | |
| | 4170-30-3 | | | |
| Cumene | 98-82-8 | 50 | 245 | X |
| Cyanides (as CN) | Varies with Compound | — | 5 | X |
| Cyanogen | 460-19-5 | 10 | — | — |
| Cyclohexane | 110-82-7 | 300 | 1050 | — |
| Cyclohexanol | 108-93-0 | 50 | 200 | — |
| Cyclohexanone | 108-94-1 | 50 | 200 | — |
| Cyclohexene | 110-83-8 | 300 | 1015 | — |
| Cyclonite | 121-82-4 | — | 1.5 | X |
| Cyclopentadiene | 542-92-7 | 75 | 200 | — |
| DDT, see Dichlorodiphenyltrichloroethane | | | | |
| DDVP, see Dichlorvos | | | | |
| 2,4-D (Dichlorophenoxyacetic acid) | 94-75-7 | — | 10 | — |
| Decaborane | 17702-41-9 | 0.05 | 0.3 | X |
| Demeton (Systox) | 8065-48-3 | — | 0.1 | X |
| Diacetone alcohol (4-Hydroxy-4-methyl-2-pentanone) | 123-42-2 | 50 | 240 | — |
| 1,2-Diaminoethane; see Ethylenediamine | | | | |
| Diazomethane | 334-88-3 | 0.2 | 0.4 | — |
| Diborane | 19287-45-7 | 0.1 | 0.1 | — |
| 1,2-Dibromo-3-chloropropane (DBCP); see § 1926.1144 | 96-12-8 | | | — |
| 1,2-Dibromoethane; see Ethylene dibromide | | | | |
| Dibutyl phosphate | 107-66-4 | 1 | 5 | — |
| Dibutyl phthalate | 84-74-2 | — | 5 | — |
| Dichloroacetylene | 7572-29-4 | (C)0.1 | (C)0.4 | — |
| o-Dichlorobenzene | 95-50-1 | (C)50 | (C)300 | — |
| p-Dichlorobenzene | 106-46-7 | 75 | 450 | — |
| 3,3′-Dichlorobenzidine; see § 1926.1107 | 91-94-1 | | | |
| Dichlorodifluoromethane | 75-71-8 | 1000 | 4950 | — |
| 1,3-Dichloro-5,5-dimethyl hydantoin | 118-52-5 | — | 0.2 | — |
| Dichlorodiphenyltrichloroethane (DDT) | 50-29-3 | — | 1 | X |
| 1,1-Dichloroethane | 75-34-3 | 100 | 400 | — |
| 1,2-Dichloroethane; see Ethylene dichloride | | | | |
| 1,2-Dichloroethylene | 540-59-0 | 200 | 790 | — |
| Dichloroethyl ether | 111-44-4 | (C)15 | (C)90 | X |
| Dichloromethane; see Methylene chloride | | | | |
| Dichloromonofluoromethane | 75-43-4 | 1000 | 4200 | — |
| 1,1-Dichloro-1-nitroethane | 594-72-9 | (C)10 | (C)60 | — |
| 1,2-Dichloropropane; see Propylene dichloride | | | | |
| Dichlorotetrafluoroethane | 76-14-2 | 1000 | 7000 | — |
| Dichlorvos (DDVP) | 62-73-7 | — | 1 | X |
| Dieldrin | 60-57-1 | — | 0.25 | X |
| Diethylamine | 109-89-7 | 25 | 75 | — |
| 2-Diethylaminoethanol | 100-37-8 | 10 | 50 | X |
| Diethylene triamine | 111-40-0 | (C)10 | (C)42 | X |
| Diethyl ether; see Ethyl ether | | | | |
| Difluorodibromomethane | 75-61-6 | 100 | 860 | — |
| Diglycidyl ether (DGE) | 2238-07-5 | (C)0.5 | (C)2.8 | — |
| Dihydroxybenzene; see Hydroquinone | | | | |
| Diisobutyl ketone | 108-83-8 | 50 | 290 | — |
| Diisopropylamine | 108-18-9 | 5 | 20 | X |
| 4-Dimethylaminoazobenzene; see § 1926.1115 | 60-11-7 | | | |
| Dimethoxymethane; see Methylal | | | | |
| Dimethyl acetamide | 127-19-5 | 10 | 35 | X |
| Dimethylamine | 124-40-3 | 10 | 18 | — |
| Dimethylaminobenzene; see Xylidine | | | | |
| Dimethylaniline (N,N-Dimethylaniline) | 121-69-7 | 5 | 25 | X |
| Dimethylbenzene; see Xylene | | | | |
| Dimethyl-1,2-dibromo- 2,2-dichloroethyl phosphate | 300-76-5 | — | 3 | — |
| Dimethylformamide | 68-12-2 | 10 | 30 | X |
| 2,6-Dimethyl-4-heptanone; see Diisobutyl ketone | | | | |
| 1,1-Dimethylhydrazine | 57-14-7 | 0.5 | 1 | X |
| Dimethylphthalate | 131-11-3 | — | 5 | — |
| Dimethyl sulfate | 77-78-3 | 1 | 5 | X |
| Dinitrobenzene (all isomers) | | | 1 | X |
| (ortho) | 528-29-0 | | | |
| (meta) | 99-65-0 | | | |
| (para) | 100-25-4 | | | |
| Dinitro-o-cresol | 534-52-1 | — | 0.2 | X |
| Dinitrotoluene | 25321-14-6 | — | 1.5 | X |
| Dioxane (Diethylene dioxide) | 123-91-1 | 100 | 360 | X |
| Diphenyl (Biphenyl) | 92-52-4 | 0.2 | 1 | — |
| Diphenylamine | 122-39-4 | — | 10 | — |
| Diphenylmethane diisocyanate; see Methylene bisphenyl isocyanate | | | | |
| Dipropylene glycol methyl ether | 34590-94-8 | 100 | 600 | X |
| Di-sec octyl phthalate (Di-(2-ethylhexyl) phthalate) | 117-81-7 | — | 5 | — |
| Emery | 12415-34-8 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Endosulfan | 115-29-7 | — | 0.1 | X |
| Endrin | 72-20-8 | — | 0.1 | X |
| Epichlorohydrin | 106-89-8 | 5 | 19 | X |
| EPN | 2104-64-5 | — | 0.5 | X |
| 1,2-Epoxypropane; see Propylene oxide | | | | |
| 2,3-Epoxy-1-propanol; see Glycidol | | | | |
| Ethane | 74-84-0 | E | | |
| Ethanethiol; see Ethyl mercaptan | | | | |
| Ethanolamine | 141-43-5 | 3 | 6 | — |
| 2-Ethoxyethanol (Cellosolve) | 110-80-5 | 200 | 740 | X |
| 2-Ethoxyethyl acetate (Cellosolve acetate) | 111-15-9 | 100 | 540 | X |
| Ethyl acetate | 141-78-6 | 400 | 1400 | — |
| Ethyl acrylate | 140-88-5 | 25 | 100 | X |
| Ethyl alcohol (Ethanol) | 64-17-5 | 1000 | 1900 | — |
| Ethylamine | 75-04-7 | 10 | 18 | — |
| Ethyl amyl ketone (5-Methyl-3-heptanone) | 541-85-5 | 25 | 130 | — |
| Ethyl benzene | 100-41-4 | 100 | 435 | — |
| Ethyl bromide | 74-96-4 | 200 | 890 | — |
| Ethyl butyl ketone (3-Heptanone) | 106-35-4 | 50 | 230 | — |
| Ethyl chloride | 75-00-3 | 1000 | 2600 | — |
| Ethyl ether | 60-29-7 | 400 | 1200 | — |
| Ethyl formate | 109-94-4 | 100 | 300 | — |
| Ethyl mercaptan | 75-08-1 | 0.5 | 1 | — |
| Ethyl silicate | 78-10-4 | 100 | 850 | — |
| Ethylene | 74-85-1 | E | | |
| Ethylene chlorohydrin | 107-07-3 | 5 | 16 | X |
| Ethylenediamine | 107-15-3 | 10 | 25 | — |
| Ethylene dibromide | 106-93-4 | (C)25 | (C)190 | X |
| Ethylene dichloride (1,2-Dichloroethane) | 107-06-2 | 50 | 200 | — |
| Ethylene glycol dinitrate | 628-96-6 | (C)0.2 | (C)1 | X |
| Ethylene glycol methyl acetate; see Methyl cellosolve acetate | | | | |
| Ethyleneimine; see § 1926.1112 | 151-56-4 | | | |
| Ethylene oxide; see § 1926.1147 | 75-21-8 | | | |
| Ethylidene chloride; see 1,1-Dichloroethane | | | | |
| N-Ethylmorpholine | 100-74-3 | 20 | 94 | X |
| Ferbam | 14484-64-1 | | | |
| Total dust | | — | 15 | — |
| Ferrovanadium dust | 12604-58-9 | — | 1 | — |
| Fibrous Glass | | | | |
| Total dust | | | | — |
| Respirable fraction | | — | | — |
| Fluorides (as F) | Varies with compound | — | 2.5 | — |
| Fluorine | 7782-41-4 | 0.1 | 0.2 | — |
| Fluorotrichloromethane (Trichlorofluoromethane) | 75-69-4 | 1000 | 5600 | — |
| Formaldehyde; see § 1926.1148 | 50-00-0 | | | |
| Formic acid | 64-18-6 | 5 | 9 | — |
| Furfural | 98-01-1 | 5 | 20 | X |
| Furfuryl alcohol | 98-00-0 | 50 | 200 | — |
| Gasoline | 8006-61-9 | | A3 | — |
| Glycerin (mist) | 56-81-5 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Glycidol | 556-52-5 | 50 | 150 | — |
| Glycol monoethyl ether; see 2-Ethoxyethanol | | | | |
| Graphite, natural, respirable dust | 7782-42-5 | (2) | (2) | (2) |
| Graphite, synthetic | | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Guthion; see Azinphos methyl | | | | |
| Gypsum | 13397-24-5 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Hafnium | 7440-58-6 | — | 0.5 | — |
| Helium | 7440-59-7 | E | | |
| Heptachlor | 76-44-8 | — | 0.5 | X |
| Heptane (n-Heptane) | 142-82-5 | 500 | 2000 | — |
| Hexachloroethane | 67-72-1 | 1 | 10 | X |
| Hexachloronaphthalene | 1335-87-1 | — | 0.2 | X |
| n-Hexane | 110-54-3 | 500 | 1800 | — |
| 2-Hexanone (Methyl n-butyl ketone) | 591-78-6 | 100 | 410 | — |
| Hexone (Methyl isobutyl ketone) | 108-10-1 | 100 | 410 | — |
| sec-Hexyl acetate | 108-84-9 | 50 | 300 | — |
| Hydrazine | 302-01-2 | 1 | 1.3 | X |
| Hydrogen | 1333-74-0 | E | | |
| Hydrogen bromide | 10035-10-6 | 3 | 10 | — |
| Hydrogen chloride | 7647-01-0 | (C)5 | (C)7 | — |
| Hydrogen cyanide | 74-90-8 | 10 | 11 | X |
| Hydrogen fluoride (as F) | 7664-39-3 | 3 | 2 | — |
| Hydrogen peroxide | 7722-84-1 | 1 | 1.4 | — |
| Hydrogen selenide (as Se) | 7783-07-5 | 0.05 | .02 | — |
| Hydrogen sulfide | 7783-06-4 | 10 | 15 | — |
| Hydroquinone | 123-31-9 | — | 2 | — |
| Indene | 95-13-6 | 10 | 45 | — |
| Indium and compounds (as In) | 7440-74-6 | — | 0.1 | — |
| Iodine | 7553-56-2 | (C)0.1 | (C)1 | — |
| Iron oxide fume | 1309-37-1 | — | 10 | — |
| Iron salts (soluble) (as Fe) | Varies with compound | — | 1 | — |
| Isoamyl acetate | 123-92-2 | 100 | 525 | — |
| Isoamyl alcohol (primary and secondary) | 123-51-3 | 100 | 360 | — |
| Isobutyl acetate | 110-19-0 | 150 | 700 | — |
| Isobutyl alcohol | 78-83-1 | 100 | 300 | — |
| Isophorone | 78-59-1 | 25 | 140 | — |
| Isopropyl acetate | 108-21-4 | 250 | 950 | — |
| Isopropyl alcohol | 67-63-0 | 400 | 980 | — |
| Isopropylamine | 75-31-0 | 5 | 12 | — |
| Isopropyl ether | 108-20-3 | 500 | 2100 | — |
| Isopropyl glycidyl ether (IGE) | 4016-14-2 | 50 | 240 | — |
| Kaolin | 1332-58-7 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Ketene | 463-51-4 | 0.5 | 0.9 | — |
| Lead, inorganic (as Pb); see 1926.62 | 7439-92-1 | | | |
| Limestone | 1317-65-3 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Lindane | 58-89-9 | — | 0.5 | X |
| Lithium hydride | 7580-67-8 | — | 0.025 | — |
| L.P.G. (Liquefied petroleum gas) | 68476-85-7 | 1000 | 1800 | |
| Magnesite | 546-93-0 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Magnesium oxide fume | 1309-48-4 | | | |
| Total particulate | | 15 | — | — |
| Malathion | 121-75-5 | | | |
| Total dust | | — | 15 | X |
| Maleic anhydride | 108-31-6 | 0.25 | | |
| Manganese compounds (as Mn) | 7439-96-5 | — | (C)5 | — |
| Manganese fume (as Mn) | 7439-96-5 | — | (C)5 | — |
| Marble | 1317-65-3 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Mercury (aryl and inorganic)(as Hg) | 7439-97-6 | | 0.1 | X |
| Mercury (organo) alkyl compounds (as Hg) | 7439-97-6 | — | 0.01 | X |
| Mercury (vapor) (as Hg) | 7439-97-6 | — | 0.1 | X |
| Mesityl oxide | 141-79-7 | 25 | 100 | — |
| Methane | 74-82-8 | E | | |
| Methanethiol; see Methyl mercaptan | | | | |
| Methoxychlor | 72-43-5 | | | |
| Total dust | | — | 15 | — |
| 2-Methoxyethanol (Methyl cellosolve) | 109-86-4 | 25 | 80 | X |
| 2-Methoxyethyl acetate (Methyl cellosolve acetate) | 110-49-6 | 25 | 120 | X |
| Methyl acetate | 79-20-9 | 200 | 610 | — |
| Methyl acetylene (Propyne) | 74-99-7 | 1000 | 1650 | — |
| Methyl acetylene-propadiene mixture (MAPP) | | 1000 | 1800 | — |
| Methyl acrylate | 96-33-3 | 10 | 35 | X |
| Methylal (Dimethoxy-methane) | 109-87-5 | 1000 | 3100 | — |
| Methyl alcohol | 67-56-1 | 200 | 260 | — |
| Methylamine | 74-89-5 | 10 | 12 | — |
| Methyl amyl alcohol; see Methyl isobutyl carbinol | | | | |
| Methyl n-amyl ketone | 110-43-0 | 100 | 465 | — |
| Methyl bromide | 74-83-9 | (C)20 | (C)80 | X |
| Methyl butyl ketone; see 2-Hexanone | | | | |
| Methyl cellosolve; see 2-Methoxyethanol | | | | |
| Methyl cellosolve acetate; see 2-Methoxyethyl acetate | | | | |
| Methylene chloride; see § 1910.1052 | | | | |
| Methyl chloroform (1,1,1-Trichloroethane) | 71-55-6 | 350 | 1900 | — |
| Methylcyclohexane | 108-87-2 | 500 | 2000 | — |
| Methylcyclohexanol | 25639-42-3 | 100 | 470 | — |
| o-Methylcyclohexanone | 583-60-8 | 100 | 460 | X |
| Methylene chloride | 75-09-2 | 500 | 1740 | — |
| Methylenedianiline (MDA) | 101-77-9 | | | |
| Methyl ethyl ketone (MEK); see 2-Butanone | | | | |
| Methyl formate | 107-31-3 | 100 | 250 | — |
| Methyl hydrazine (Monomethyl hydrazine) | 60-34-4 | (C)0.2 | (C)0.35 | X |
| Methyl iodide | 74-88-4 | 5 | 28 | X |
| Methyl isoamyl ketone | 110-12-3 | 100 | 475 | — |
| Methyl isobutyl carbinol | 108-11-2 | 25 | 100 | X |
| Methyl isobutyl ketone; see Hexone | | | | |
| Methyl isocyanate | 624-83-9 | 0.02 | 0.05 | X |
| Methyl mercaptan | 74-93-1 | 0.5 | 1 | — |
| Methyl methacrylate | 80-62-6 | 100 | 410 | — |
| Methyl propyl ketone; see 2-Pentanone | | | | |
| Methyl silicate | 681-84-5 | (C)5 | (C)30 | — |
| alpha-Methyl styrene | 98-83-9 | (C)100 | (C)480 | — |
| Methylene bisphenyl isocyanate (MDI) | 101-68-8 | (C)0.02 | (C)0.2 | — |
| Mica; see Silicates | | | | |
| Molybdenum (as Mo) | 7439-98-7 | | | |
| Soluble compounds | | — | 5 | — |
| Insoluble compounds | | | | |
| Total dust | | — | 15 | — |
| Monomethyl aniline | 100-61-8 | 2 | 9 | X |
| Monomethyl hydrazine; see Methyl hydrazine | | | | |
| Morpholine | 110-91-8 | 20 | 70 | X |
| Naphtha (Coal tar) | 8030-30-6 | 100 | 400 | — |
| Naphthalene | 91-20-3 | 10 | 50 | — |
| alpha-Naphthylamine; see § 1926.1104 | 134-32-7 | | | |
| beta-Naphthylamine; see § 1926.1109 | 91-59-8 | | | — |
| Neon | 7440-01-9 | E | | |
| Nickel carbonyl (as Ni) | 13463-39-3 | 0.001 | 0.007 | — |
| Nickel, metal and insoluble compounds (as Ni) | 7440-02-0 | — | 1 | — |
| Nickel, soluble compounds (as Ni) | 7440-02-0 | — | 1 | — |
| Nicotine | 54-11-5 | — | 0.5 | X |
| Nitric acid | 7697-37-2 | 2 | 5 | — |
| Nitric oxide | 10102-43-9 | 25 | 30 | — |
| p-Nitroaniline | 100-01-6 | 1 | 6 | X |
| Nitrobenzene | 98-95-3 | 1 | 5 | X |
| p-Nitrochlorobenzene | 100-00-5 | — | 1 | X |
| 4-Nitrodiphenyl; see § 1926.1103 | 92-93-3 | | | |
| Nitroethane | 79-24-3 | 100 | 310 | — |
| Nitrogen | 7727-37-9 | E | | |
| Nitrogen dioxide | 10102-44-0 | (C)5 | (C)9 | — |
| Nitrogen trifluoride | 7783-54-2 | 10 | 29 | — |
| Nitroglycerin | 55-63-0 | (C)0.2 | (C)2 | X |
| Nitromethane | 75-52-5 | 100 | 250 | — |
| 1-Nitropropane | 108-03-2 | 25 | 90 | — |
| 2-Nitropropane | 79-46-9 | 25 | 90 | — |
| N-Nitrosodimethylamine; see § 1926.1116 | 62-79-9 | | | — |
| Nitrotoluene (all isomers) | | 5 | 30 | X |
| o-isomer | 88-72-2; | | | |
| m-isomer | 99-08-1; | | | |
| p-isomer | 99-99-0 | | | |
| Nitrotrichloromethane; see Chloropicrin | | | | |
| Nitrous oxide | 10024-97-2 | E | | |
| Octachloronaphthalene | 2234-13-1 | — | 0.1 | X |
| Octane | 111-65-9 | 400 | 1900 | — |
| Oil mist, mineral | 8012-95-1 | — | 5 | — |
| Osmium tetroxide (as Os) | 20816-12-0 | — | 0.002 | — |
| Oxalic acid | 144-62-7 | — | 1 | — |
| Oxygen difluoride | 7783-41-7 | 0.05 | 0.1 | — |
| Ozone | 10028-15-6 | 0.1 | 0.2 | — |
| Paraquat, respirable dust | 4685-14-7; | — | 0.5 | X |
| | 1910-42-5; | | | |
| | 2074-50-2 | | | |
| Parathion | 56-38-2 | — | 0.1 | X |
| Particulates not otherwise regulated | | | | |
| Total dust organic and inorganic | | — | 15 | — |
| PCB; see Chlorodiphenyl (42% and 54% chlorine) | | | | |
| Pentaborane | 19624-22-7 | 0.005 | 0.01 | — |
| Pentachloronaphthalene | 1321-64-8 | — | 0.5 | X |
| Pentachlorophenol | 87-86-5 | — | 0.5 | X |
| Pentaerythritol | 115-77-5 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Pentane | 109-66-0 | 500 | 1500 | — |
| 2-Pentanone (Methyl propyl ketone) | 107-87-9 | 200 | 700 | — |
| Perchloroethylene (Tetrachloroethylene) | 127-18-4 | 100 | 670 | — |
| Perchloromethyl mercaptan | 594-42-3 | 0.1 | 0.8 | — |
| Perchloryl fluoride | 7616-94-6 | 3 | 13.5 | — |
| Petroleum distillates (Naphtha)(Rubber Solvent) | | | A3 | — |
| Phenol | 108-95-2 | 5 | 19 | X |
| p-Phenylene diamine | 106-50-3 | — | 0.1 | X |
| Phenyl ether, vapor | 101-84-8 | 1 | 7 | — |
| Phenyl ether-biphenyl mixture, vapor | | 1 | 7 | — |
| Phenylethylene; see Styrene | | | | |
| Phenyl glycidyl ether (PGE) | 122-60-1 | 10 | 60 | — |
| Phenylhydrazine | 100-63-0 | 5 | 22 | X |
| Phosdrin (Mevinphos) | 7786-34-7 | — | 0.1 | X |
| Phosgene (Carbonyl chloride) | 75-44-5 | 0.1 | 0.4 | — |
| Phosphine | 7803-51-2 | 0.3 | 0.4 | — |
| Phosphoric acid | 7664-38-2 | — | 1 | — |
| Phosphorus (yellow) | 7723-14-0 | — | 0.1 | — |
| Phosphorus pentachloride | 10026-13-8 | — | 1 | — |
| Phosphorus pentasulfide | 1314-80-3 | — | 1 | — |
| Phosphorus trichloride | 7719-12-2 | 0.5 | 3 | — |
| Phthalic anhydride | 85-44-9 | 2 | 12 | — |
| Picric acid | 88-89-1 | — | 0.1 | X |
| Pindone (2-Pivalyl-1,3-indandione) | 83-26-1 | — | 0.1 | — |
| Plaster of Paris | 26499-65-0 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Platinum (as Pt) | 7440-06-4 | | | |
| Metal | | — | — | — |
| Soluble salts | | — | 0.002 | — |
| Polytetrafluoroethylene decomposition products | | | A2 | |
| Portland cement | 65997-15-1 | | | |
| Total dust | | — | 15 | — |
| Respirable fraction | | 5 | | — |
| Propane | 74-98-6 | E | | |
| Propargyl alcohol | 107-19-7 | 1 | — | X |
| beta-Propriolactone; see § 1926.1113 | 57-57-8 | | | |
| n-Propyl acetate | 109-60-4 | 200 | 840 | — |
| n-Propyl alcohol | 71-23-8 | 200 | 500 | — |
| n-Propyl nitrate | 627-13-4 | 25 | 110 | — |
| Propylene dichloride | 78-87-5 | 75 | 350 | — |
| Propylene imine | 75-55-8 | 2 | 5 | X |
| Propylene oxide | 75-56-9 | 100 | 240 | — |
| Propyne; see Methyl acetylene | | | | |
| Pyrethrum | 8003-34-7 | — | 5 | — |
| Pyridine | 110-86-1 | 5 | 15 | — |
| Quinone | 106-51-4 | 0.1 | 0.4 | — |
| RDX; see Cyclonite | | | | |
| Rhodium (as Rh), metal fume and insoluble compounds | 7440-16-6 | — | 0.1 | — |
| Rhodium (as Rh), soluble compounds | 7440-16-6 | — | 0.001 | — |
| Ronnel | 299-84-3 | — | 10 | — |
| Rotenone | 83-79-4 | — | 5 | — |
| Rouge | | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Selenium compounds (as Se) | 7782-49-2 | — | 0.2 | — |
| Selenium hexafluoride (as Se) | 7783-79-1 | 0.05 | 0.4 | — |
| Silica, amorphous, precipitated and gel | 112926-00-8 | (2) | (2) | (2) |
| Silica, amorphous, diatomaceous earth, containing less than 1% crystalline silica | 61790-53-2 | (2) | (2) | (2) |
| Silica, crystalline, respirable dust | | | | |
| Cristobalite; see 1926.1153 | 14464-46-1 | | | |
| Quartz; see 1926.11535 | 14808-60-7 | | | |
| Tripoli (as quartz); see 1926.11535 | 1317-95-9 | | | |
| Tridymite; see 1926.1153 | 15468-32-3 | | | |
| Silica, fused, respirable dust | 60676-86-0 | (2) | (2) | (2) |
| Silicates (less than 1% crystalline silica) | | | | |
| Mica (respirable dust) | 12001-26-2 | (2) | (2) | (2) |
| Soapstone, total dust | | (2) | (2) | (2) |
| Soapstone, respirable dust | | (2) | (2) | (2) |
| Talc (containing asbestos); use asbestos limit; see § 1926.1101 | | | | |
| Talc (containing no asbestos), respirable dust | 14807-96-6 | (2) | (2) | (2) |
| Tremolite, asbestiform; see § 1926.1101 | | | | |
| Silicon carbide | 409-21-2 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Silver, metal and soluble compounds (as Ag) | 7440-22-4 | — | 0.01 | — |
| Soapstone; see Silicates | | | | |
| Sodium fluoroacetate | 62-74-8 | — | 0.05 | X |
| Sodium hydroxide | 1310-73-2 | — | 2 | — |
| Starch | 9005-25-8 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Stibine | 7803-52-3 | 0.1 | 0.5 | — |
| Stoddard solvent | 8052-41-3 | 200 | 1150 | — |
| Strychnine | 57-24-9 | — | 0.15 | — |
| Styrene | 100-42-5 | (C)100 | (C)420 | — |
| Sucrose | 57-50-1 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Sulfur dioxide | 7446-09-5 | 5 | 13 | — |
| Sulfur hexafluoride | 2551-62-4 | 1000 | 6000 | — |
| Sulfuric acid | 7664-93-9 | — | 1 | — |
| Sulfur monochloride | 10025-67-9 | 1 | 6 | — |
| Sulfur pentafluoride | 5714-22-7 | 0.025 | 0.25 | — |
| Sulfuryl fluoride | 2699-79-8 | 5 | 20 | — |
| Systox, see Demeton | | | | |
| 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) | 93-76-5 | — | 10 | — |
| Talc; see Silicates— | | | | |
| Tantalum, metal and oxide dust | 7440-25-7 | — | 5 | — |
| TEDP (Sulfotep) | 3689-24-5 | — | 0.2 | X |
| Teflon decomposition products | | | A2 | |
| Tellurium and compounds (as Te) | 13494-80-9 | — | 0.1 | — |
| Tellurium hexafluoride (as Te) | 7783-80-4 | 0.02 | 0.2 | — |
| Temephos | 3383-96-8 | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| TEPP (Tetraethyl pyrophosphate) | 107-49-3 | — | 0.05 | X |
| Terphenyls | 26140-60-3 | (C)1 | (C)9 | — |
| 1,1,1,2-Tetrachloro-2,2-difluoroethane | 76-11-9 | 500 | 4170 | — |
| 1,1,2,2-Tetrachloro-1,2-difluoroethane | 76-12-0 | 500 | 4170 | — |
| 1,1,2,2-Tetrachloroethane | 79-34-5 | 5 | 35 | X |
| Tetrachloroethylene; see Perchloroethylene | | | | |
| Tetrachloromethane; see Carbon tetrachloride | | | | |
| Tetrachloronaphthalene | 1335-88-2 | — | 2 | X |
| Tetraethyl lead (as Pb) | 78-00-2 | — | 0.1 | X |
| Tetrahydrofuran | 109-99-9 | 200 | 590 | — |
| Tetramethyl lead, (as Pb) | 75-74-1 | — | 0.15 | X |
| Tetramethyl succinonitrile | 3333-52-6 | 0.5 | 3 | X |
| Tetranitromethane | 509-14-8 | 1 | 8 | — |
| Tetryl (2,4,6-Trinitrophenylmethylnitramine) | 479-45-8 | — | 1.5 | X |
| Thallium, soluble compounds (as Tl) | 7440-28-0 | — | 0.1 | X |
| Thiram | 137-26-8 | — | 5 | — |
| Tin, inorganic compounds (except oxides) (as Sn) | 7440-31-5 | — | 2 | — |
| Tin, organic compounds (as Sn) | 7440-31-5 | — | 0.1 | — |
| Tin oxide (as Sn) | 21651-19-4 | — | — | — |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Titanium dioxide | 13463-67-7 | | | |
| Total dust | | — | | — |
| Toluene | 108-88-3 | 200 | 750 | — |
| Toluene-2,4-diisocyanate (TDI) | 584-84-9 | (C)0.02 | (C)0.14 | — |
| o-Toluidine | 95-53-4 | 5 | 22 | X |
| Toxaphene; see Chlorinated camphene | | | | |
| Tremolite; see Silicates | | | | |
| Tributyl phosphate | 126-73-8 | — | 5 | — |
| 1,1,1-Trichloroethane; see Methyl chloroform | | | | |
| 1,1,2-Trichloroethane | 79-00-5 | 10 | 45 | X |
| Trichloroethylene | 79-01-6 | 100 | 535 | — |
| Trichloromethane; see Chloroform | | | | |
| Trichloronaphthalene | 1321-65-9 | — | 5 | X |
| 1,2,3-Trichloropropane | 96-18-4 | 50 | 300 | — |
| 1,1,2-Trichloro-1,2,2-trifluoroethane | 76-13-1 | 1000 | 7600 | — |
| Triethylamine | 121-44-8 | 25 | 100 | — |
| Trifluorobromomethane | 75-63-8 | 1000 | 6100 | — |
| Trimethyl benzene | 25551-13-7 | 25 | 120 | — |
| 2,4,6-Trinitrophenol; see Picric acid | | | | |
| 2,4,6-Trinitrophenylmethylnitramine; see Tetryl | | | | |
| 2,4,6-Trinitrotoluene (TNT) | 118-96-7 | — | 1.5 | X |
| Triorthocresyl phosphate | 78-30-8 | — | 0.1 | — |
| Triphenyl phosphate | 115-86-6 | — | 3 | — |
| Tungsten (as W) | 7440-33-7 | | | |
| Insoluble compounds | | — | 5 | — |
| Soluble compounds | | — | 1 | — |
| Turpentine | 8006-64-2 | 100 | 560 | — |
| Uranium (as U) | 7440-61-1 | | | |
| Soluble compounds | | — | 0.2 | — |
| Insoluble compounds | | — | 0.2 | — |
| Vanadium | 1314-62-1 | | | |
| Respirable dust (as V2 O5) | | — | (C)0.5 | — |
| Fume (as V2 O5) | | — | (C)0.1 | — |
| Vegetable oil mist | | | | |
| Total dust | | — | | — |
| Respirable fraction | | — | | — |
| Vinyl benzene; see Styrene | | | | |
| Vinyl chloride; see § 1926.1117 | 75-01-4 | | | |
| Vinyl cyanide; see Acrylonitrile | | | | |
| Vinyl toluene | 25013-15-4 | 100 | 480 | — |
| Warfarin | 81-81-2 | — | 0.1 | — |
| Xylenes (o-, m-, p-isomers) | 1330-20-7 | 100 | 435 | — |
| Xylidine | 1300-73-8 | 5 | 25 | X |
| Yttrium | 7440-65-5 | — | 1 | — |
| Zinc chloride fume | 7646-85-7 | — | 1 | — |
| Zinc oxide fume | 1314-13-2 | — | 5 | — |
| Zinc oxide | 1314-13-2 | | | |
| Total dust | | — | 15 | — |
| Respirable fraction | | — | 5 | — |
| Zirconium compounds (as Zr) | 7440-67-7 | — | 5 | |

Table 1 to § 1926.55—Permissible Exposure Limits for Airborne Contaminants

| Substance | mppcf(j) |
| --- | --- |
| SILICA: | |
| Crystalline | 250(k) |
| Quartz. Threshold Limit calculated from the formula(p) | % SiO2 + 5 |
| Cristobalite | |
| Amorphous, including natural diatomaceous earth | 20 |
| SILICATES (less than 1% crystalline silica) | |
| Mica | 20 |
| Portland cement | 50 |
| Soapstone | 20 |
| Talc (non-asbestiform) | 20 |
| Talc (fibrous), use asbestos limit | — |
| Graphite (natural) | 15 |
| Inert or Nuisance Particulates:(m) | 50 (or 15 mg/m3 whichever is the smaller) of total dust <1% SiO2 |
| [Inert or Nuisance Dusts includes all mineral, inorganic, and organic dusts as indicated by examples in TLV's appendix D] | |
| Conversion factors | |
| mppcf × 35.3 = million particles per cubic meter = particles per c.c. | |
| Footnotes to Tables 1 and 2 of this section: |
| 1 [Reserved] |
| 2 See Table 2 of this section. |
| 3 Use Asbestos Limit § 1926.1101. |
| 4 [Reserved] |
| 5 See Table 2 of this section for the exposure limit for any operations or sectors where the exposure limit in § 1926.1153 is stayed or is otherwise not in effect. |
| * An “X” designation in the “Skin Designation” column indicates that the substance is a dermal hazard. |
| a Parts of vapor or gas per million parts of contaminated air by volume at 25 °C and 760 torr. |
| b Milligrams of substance per cubic meter of air. When entry is in this column only, the value is exact; when listed with a ppm entry, it is approximate. |
| c [Reserved] |
| d The CAS number is for information only. Enforcement is based on the substance name. For an entry covering more than one metal compound, measured as the metal, the CAS number for the metal is given—not CAS numbers for the individual compounds. |
| e-f [Reserved] |
| g For sectors excluded from § 1926.1128 the limit is 10 ppm TWA. |
| h-i [Reserved] |
| j Millions of particles per cubic foot of air, based on impinger samples counted by light-field techniques. |
| k 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. |
| l [Reserved] |
| m Covers all organic and inorganic particulates not otherwise regulated. Same as Particulates Not Otherwise Regulated. |
| n If the exposure limit in § 1926.1126 is stayed or is otherwise not in effect, the exposure limit is a ceiling of 0.1 mg/m3. |
| o If the exposure limit in § 1926.1126 is stayed or is otherwise not in effect, the exposure limit is 0.1 mg/m3 (as CrO3) as an 8-hour TWA. |
| p This standard applies to any operations or sectors for which the respirable crystalline silica standard, 1926.1153, is stayed or otherwise is not in effect. |
| q This standard applies to any operations or sectors for which the beryllium standard, 1926.1124, is stayed or otherwise is not in effect. |
| The 1970 TLV uses letter designations instead of a numerical value as follows: |
| A1 [Reserved] |
| A2 Polytetrafluoroethylene decomposition products. Because these products decompose in part by hydrolysis in alkaline solution, they can be quantitatively determined in air as fluoride to provide an index of exposure. No TLV is recommended pending determination of the toxicity of the products, but air concentrations should be minimal. |
| A3 Gasoline and/or Petroleum Distillates. The composition of these materials varies greatly and thus a single TLV for all types of these materials is no longer applicable. The content of benzene, other aromatics and additives should be determined to arrive at the appropriate TLV. |
| E Simple asphyxiants. The limiting factor is the available oxygen which shall be at least 19.5% and be within the requirements addressing explosion in part 1926. |

Table 2 to § 1926.55—Mineral Dusts

[39 FR 22801, June 24, 1974, as amended at 51 FR 37007, Oct. 17, 1986; 52 FR 46312, Dec. 4, 1987; 58 FR 35089, June 30, 1993; 61 FR 9249, 9250, Mar. 7, 1996; 61 FR 56856, Nov. 4, 1996; 62 FR 1619, Jan. 10, 1997; 71 FR 10381, Feb. 28, 2006; 71 FR 36009, June 23, 2006; 81 FR 16875, Mar. 25, 2016; 81 FR 31168, May 18, 2016; 81 FR 60273, Sept. 1, 2016; 82 FR 2750, Jan. 9, 2017; 84 FR 21576, May 14, 2019]