The primary sources of exposure criteria for workplace inhalation exposures are the following: (1) NIOSH recommended exposure limits (RELs),42 (2) the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs®),43 and (3) the U.S. Department of Labor, OSHA permissible exposure limits (PELs).44 These exposure criteria are for diisocyanate monomers. In July 1992, the 11th Circuit Court of Appeals vacated the 1989 OSHA PEL Air Contaminants Standard. OSHA is currently enforcing the 1971 standards which are listed as transitional values in the current Code of Federal Regulations; however, some states operating their own OSHA approved job safety and health programs can have lower limits. Table 1 contains a comparison of the respective NIOSH RELs, ACGIH TLVs, OSHA PELs, and United Kingdom Health and Safety Executive (UK-HSE) exposure criteria for the isocyanates. The UK-HSE has taken a different approach, i.e., developing a non-specific standard based on the total number of reactive isocyanate groups (TRIG) in a volume of air.45 The standards and limits listed in Table 1 are based on a general standard that isocyanate exposures should not exceed an 8-hour time-weighted average (TWA) exposure of 5 parts per billion (ppb), or a short term or ceiling exposure of 20 ppb.
Table 1: NIOSH, ACGIH, OSHA, and UK-HSE Exposure Criteria for Isocyanates Isocyanate Species
1
2
Exposure Criteria - Full-shift TWAs Micrograms per cubic meter of air
Exposure Criteria - Short Term or Ceiling Limits Micrograms per cubic meter of air
NIOSH REL
ACGIH TLV
UK-HSE
NIOSH REL Ceiling
ACGIH TLV-STEL
UK-HSE Ceiling
OSHA PEL Ceiling
TDI
CA-LFC1
36
None
None
140
None
140
MDI
50
51
None
200
None
None
200
HDI
35
34
None
140
None
None
None
HMDI
None
54
None
210
None
None
None
IPDI
45
45
None
180
None
None
None
NDI
40
None
None
170
None
None
None
TRIG2
None
None
20
None
None
70
None
NIOSH considers TDI to be an occupational carcinogen (CA) and recommends that exposures be reduced to the lowest feasible concentration (LFC). TRIG - total reactive isocyanate group.
Table II. Comparison of NIOSH and OSHA Isocyanate Methods NIOSH 5521
NIOSH 5522
NIOSH 2535
OSHA 42/47
PROPOSED NEW NIOSH1
a) Monomers
TDI, MDI, HDI, NDI, HMDI2
TDI, MDI, HDI, NDI, HMDI,2 IPDI 2
TDI, HDI
42 TDI, HDI 47 MDI
TDI, HDI, MDI, NDI,2 HMDI,2 IPDI 2
b) Oligomers
HDI
TDI, MDI, HDI
None
None
HDI, MDI,2 TDI2
impinger
impinger
coated glass wool/opaque tube
coated GFF
impinger; GFF; impinger+GFF
Isocyanate
Sampler
1/15/98
119
NIOSH Manual of Analytical Methods
