Isocyanates (HDI, MDI, TDI): why the filter decides the result
This page explains what the rules and common practice call for. Every site is different, and what a survey or a filing needs at your facility is something we work out with you after we see it.
With isocyanates, the wrong filter does not give you a wrong answer. It gives you no answer, and you do not find out until the laboratory report comes back.
HDI, MDI and TDI are the common abbreviations for three isocyanates, each of which is a different chemical.
Why this one is different
Isocyanates are reactive, so they will not sit on a standard filter and wait for the laboratory. They break down, and they react with whatever is around them.
Sampling therefore requires derivatizing media: a filter or an impinger (a small bubbler filled with liquid) treated with a reagent. The reagent reacts with the isocyanate on contact and forms a stable compound the laboratory can measure.
Get that wrong and you get a clean result from a dirty plant. Standard cassettes and untreated filters work for most other substances but not here, and nothing in the field tells you the sample has failed.
Where we find them
- Two-part polyurethane spray coatings. Auto refinish, aerospace topcoats and industrial maintenance coatings. HDI and its oligomers (short chains of linked HDI molecules).
- Spray polyurethane foam for insulation and roofing. MDI.
- Flexible foam manufacturing, adhesives, sealants and binders. TDI and MDI.
- Foundry core binders. Phenolic urethane systems. MDI.
- Thermal decomposition of cured polyurethane. Welding, cutting or grinding on coated or foam-backed material releases isocyanates from material that was inert a moment earlier, and this source is widely missed.
Exposure limits
| OSHA PEL | ACGIH TLV | |
|---|---|---|
| 2,4-TDI (CAS 584-84-9) | Ceiling limit 0.02 ppm (0.14 mg/m³) | 0.001 ppm shift average, inhalable fraction and vapor, with a 0.005 ppm STEL |
| MDI | Ceiling limit 0.02 ppm (0.2 mg/m³) | 0.005 ppm |
| HDI (CAS 822-06-0) | None. No PEL, and no entry in OSHA's air contaminant limits (Table Z). | 0.005 ppm |
The PEL is OSHA's permissible exposure limit, and the TLV is the threshold limit value published by ACGIH. In the table, ppm means parts per million, mg/m³ means milligrams per cubic meter of air, and STEL means short-term exposure limit. A ceiling limit is a concentration that must not be exceeded at any time.
Two things in that table deserve more attention than they usually get.
The TDI gap is 20-fold. OSHA's ceiling limit, the one it can enforce, is 0.02 ppm, while the ACGIH TLV is 0.001 ppm.
Note also that TDI exists as two isomers, 2,4 and 2,6, and commercial TDI is typically an 80/20 mixture of the two. OSHA's Table Z entry, however, covers 2,4-TDI only, so a survey report should state which isomers were measured, not only “TDI.”
HDI has no OSHA limit at all. It is among the most widely sprayed isocyanates in industrial coatings, yet OSHA has no standard specific to it and no Table Z entry.
That does not mean you have no obligation. The obligation comes through the General Duty Clause and, where workers wear respirators, through OSHA's respiratory protection standard, not through a numerical PEL. Where a recognized health-based limit exists and a feasible control is available, “there's no legal limit” is not a defense.
A third point sits outside the table: isocyanates are sensitizers. Once a worker is sensitized, later exposures far below any limit can trigger a severe asthma reaction.
That changes the logic of the whole program. With a sensitizer, the goal is not to show that you meet a number but to find and control exposure before someone becomes sensitized. A sensitized worker is often a permanent job placement problem and a workers' compensation claim.
How we sample for them
| Sampling media | Derivatizing media. These are treated glass fiber filters (1-2PP or equivalent), or treated impingers, depending on the method and the isocyanate |
| Analytical methods | OSHA 5002. The current validated method. It covers HDI, HDI homopolymer, IPDI, HMDI, MDI, polymeric MDI, 2,4-TDI and 2,6-TDI · NIOSH 5525 (total isocyanate group. It measures monomer and oligomer forms) · ISO 17734-1:2013 |
| Sample type | Personal samples in the worker's breathing zone. Short-term samples are essential where ceiling limits apply, because a full-shift average cannot demonstrate compliance with a ceiling limit |
| Handling | Time-sensitive. The media have a limited holding time before analysis, and they need proper storage from the moment sampling stops. |
Monomer or oligomer is the question that determines whether the numbers mean anything. Most industrial spray coatings today are formulated with polyisocyanate oligomers, the linked chains, and not with free monomer, the single molecule. A method that measures only the monomer will understate exposure by a wide margin.
Method selection is therefore a real decision, not paperwork. NIOSH 5525 measures the total reactive isocyanate group, so it captures the oligomer forms as well as the monomer, and OSHA 5002 likewise covers HDI homopolymer and polymeric MDI along with the monomers. A written scope of work should name the method and the forms it captures.
A note on method numbers. OSHA methods 42 and 47 were the standard isocyanate methods for years: Method 47 covered MDI, and Method 42 covered HDI and both TDI isomers. OSHA withdrew both in April 2021, and the method PDFs now carry a “Provided for Historical Reference Only” header.
The current fully validated method is OSHA 5002 (February 2021), and it is the one to look for on a proposal or a laboratory report.
What makes isocyanate sampling difficult
The sampling media have to be ordered and handled with care, and they are not what a general sampling kit carries. Beyond that, the work involves three judgment calls:
- Which forms of isocyanate to target
- When a short-term sample is required and not a full-shift sample
- How to get time-sensitive media from the plant to the laboratory within their holding time
Talk to us about isocyanate air sampling
Ron Fleming has done this work himself since 1993. You talk directly to the person who does the job. No account managers. No call center. Our prices are well below the competition. Tell us what is going on and we will send a written scope of work and a price, at no cost.
Talk with us. No cost.References
The regulations and standards behind this page, with links to the official text, so you can check the source yourself.
- OSHA 29 CFR 1910.134 Respiratory Protection