In brief
Polyurethane foam, two-component paints and varnishes and some adhesives contain isocyanates; these substances can cause sensitisation via the respiratory tract and skin, in other words isocyanate asthma. Because a sensitised employee can have an attack even at exposures far below the limit value, control at source comes first, followed by the right respiratory protection and health surveillance managed by the physician. In spraying work, a filter mask is often not enough; supplied-air protection is preferred.
Where does isocyanate exposure begin?
Isocyanates are reactive chemicals that react with polyols to form polyurethane. The vapour, mist or dust released into the air during this reaction and before it is complete is the source of isocyanate exposure. Once the product has fully cured, for example in a finished seat foam or dried paint, the risk largely disappears. The hazard therefore lies not in the product itself but in operations such as mixing, pouring, spraying, heating and grinding.
In the automotive supply industry in Bursa and the surrounding provinces, seat and headliner foam production, paint lines, vehicle repair paint shops and insulation work are the tasks where isocyanates are most often encountered. The table below summarises how exposure occurs by type of work.
| Work | Where is the isocyanate? | How does exposure occur? |
|---|---|---|
| Polyurethane foam moulding (seats, foam, moulded parts) | Component containing MDI or TDI | Mixing head, mould opening, mould cleaning |
| Two-component paint and varnish | Hardener (usually HDI-based) | Spray mist, mixing, gun cleaning |
| Polyurethane adhesive and sealant | One- or two-component product | Hot application, spreading over a large surface, skin contact |
| Spray polyurethane insulation | Component containing MDI | Spray mist, build-up in an enclosed volume |
| Heating, cutting or welding cured polyurethane | Decomposition products released by heat | Hot wire cutting, contact with flame or welding |
The safest way to tell which product contains isocyanate is the safety data sheet. We explained how to read the sections of the form in our article on the safety data sheet.
MDI, TDI and HDI: why does vapour pressure matter?
The three abbreviations you will hear most often on site describe three isocyanate groups that behave differently:
- TDI (toluene diisocyanate): Its volatility is relatively high. It evaporates even at room temperature; in flexible foam production it easily builds up in the air during open mixing and pouring.
- MDI (methylene diphenyl diisocyanate): Its volatility is low, which is why it is often thought to be "safer". However, when heated, sprayed or turned into a fine mist, it becomes inhalable. The risk rises markedly in hot moulds and spray insulation.
- HDI and similar aliphatic isocyanates: Found in the hardeners of automotive paints, usually as prepolymers (oligomers). Little is inhaled as vapour, but much is inhaled as droplets in the spray mist.
For the same reason, the critical moments for health in polyurethane foam production depend on the operation: the minutes when the mixing head is cleaned, the mould is opened while hot and the flash on fresh foam is handled by hand. When planning measurements, these short but intense moments of exposure should be targeted separately; the shift average alone may not show the full picture.
The practical conclusion is that the risk comes from the operation more than from the type of isocyanate used. A low-volatility MDI product, when sprayed, can be as dangerous as TDI poured in the open. In the risk assessment, write down not the product name but how the product is applied.
Annex 1 of the Regulation on Health and Safety Measures in Work with Chemical Agents lists an 8-hour limit value of 0.05 mg/m³ (0.005 ppm) for MDI (methylene bisphenyl isocyanate). This value shows how low the limit values are; however, it does not mean a safe level for a sensitised employee.
Isocyanate asthma: how does sensitisation develop?
Isocyanates are both irritants and sensitisers. The first effects are watery eyes, a runny nose, a burning throat and coughing. The real danger, however, is sensitisation, which can develop over months or years: the immune system "recognises" the substance, and afterwards even a very small exposure can trigger an asthma attack with wheezing, shortness of breath and chest tightness.
Features to watch for in isocyanate asthma are:
- Delayed symptoms. Symptoms may appear hours after work, in the evening or at night. The employee may not link them to work.
- Difference between work days and days off. Symptoms that ease at weekends and on leave and worsen on return to work are an important clue.
- Skin route. Bare-hand contact with the hardener can also contribute to sensitisation; working without gloves is a risk even if breathing is protected.
- Risk of irreversibility. If exposure continues after sensitisation has occurred, the asthma can become permanent.
Products whose safety data sheet and label carry H334 (may cause allergy or asthma symptoms or breathing difficulties if inhaled) and H317 (may cause an allergic skin reaction) belong to this group. For the diagnostic process, PEF monitoring and comparing work days with days off, see our article on occupational asthma.
Control at source: substitution, closed systems and extraction
The mask is the last step. The following questions are asked first:
- Can an isocyanate-free system be used? In some applications, water-based or isocyanate-free products can be tried. Because a product change will affect quality, the decision is made together with production and quality.
- Can the process be enclosed? In foam production, a closed mixing head and automatic dosing reduce exposure markedly compared with mixing in an open bucket.
- Is spraying done in a booth? Two-component paint is applied only in a booth with working extraction. No spraying is done outside the booth, even for "a small touch-up". For booth and ventilation details, see our article on paint booth and painter risks.
- Is there local exhaust ventilation at the mould-opening and cleaning points? The moment the hot mould is opened is one of the moments of highest exposure on a foam line.
- Where does uncured product wait? Fresh foam and painted parts should not be left to cure in an area where many employees work.
- Has hot work been separated? If welding, hot wire cutting or flame work is to be done on a polyurethane-coated part, separate measures are planned for the decomposition products.
Cleaning and spill response are also written into the plan: vapour from the solvent used to clean guns and heads, spilled hardener spreading, and keeping waste containers closed are frequently overlooked points. For the method of prioritising chemicals, see our article on chemical risk assessment.
Mask selection: when is a filter mask not enough?
Respiratory protection is selected according to section 8 of the safety data sheet and the work being done. The general approach is:
| Work | Recommended respiratory protection | Note |
|---|---|---|
| Spraying two-component paint | Supplied-air mask or hood | This is the first choice even inside the booth |
| Short mixing and dosing outside the booth | Mask with gas/vapour and particle filter (e.g. A2P3) | The filter change time is set in advance |
| Supervising an enclosed foam line | Often not needed if the process is enclosed | Decided according to the measurement result and risk assessment |
| Spray polyurethane insulation | Supplied-air protection | Build-up in an enclosed volume is assessed separately |
| Spill and breakdown response | Supplied-air or self-contained breathing apparatus | The response team is trained in advance |
There are two reasons why filter masks are of limited use against isocyanates. First, because there is no smell warning, the employee cannot tell when the filter is saturated; so it is changed according to a set schedule, not "when you smell it". Second, at the high concentrations in spraying, even a small leak in the face-to-mask fit means significant exposure. For filter types and colour codes, see our article on gas mask filter colour codes.
Skin protection is treated just as seriously. Glove material is selected according to the safety data sheet; in automotive painting, nitrile gloves are usually recommended, and thin latex gloves are not suitable. Coveralls should cover the painter's arms and neck.
The place of isocyanates in health surveillance
Art. 12 of the Regulation on Chemical Agents requires employees found in the risk assessment to be at risk to their health to be placed under appropriate health surveillance, with individual health and exposure records kept. If a disease or adverse effect is found, the employee is informed, the risk assessment is reviewed and, if necessary, the employee is moved to work without exposure. For employees working with isocyanates, this provision is applied as follows:
- Pre-employment examination. The physician asks about a history of asthma, allergic rhinitis and previous isocyanate-related complaints. They may request a pulmonary function test (PFT) to have a baseline value.
- Periodic monitoring. The legislation does not set a fixed frequency for PFTs or questionnaires; the physician decides according to the level of exposure. Regularly using a short symptom questionnaire is a practical way of catching delayed symptoms. For interpreting the test, see our article on the pulmonary function test.
- Symptom reporting. Employees are told that they should also report coughing or wheezing that starts in the evening. Early reporting ensures sensitisation is noticed before it becomes permanent.
- Suspected sensitisation. The physician may recommend removing the employee from exposure and starts the occupational disease process. For these steps, see our article on the physician's role when an occupational disease is suspected.
Health surveillance and workplace measurement feed into each other: measurement results show who should be monitored more often, while health findings show which operation needs to be revisited. For other risks in the automotive and supply industry, see our automotive supply industry page. In departments using isocyanates, we plan health surveillance and risk assessment work together.
Frequently Asked Questions
Is cured polyurethane foam dangerous?
Fully cured foam does not cause isocyanate exposure in normal use. The risk arises from decomposition products released when the foam is heated, cut with a hot wire or burned. These tasks are assessed separately.
Which mask should be used for two-component paint?
For spraying, the first choice is a supplied-air mask or hood. For short mixing tasks, a mask with a gas/vapour and particle filter can be used; the filter is changed according to a set schedule without waiting for a smell.
Does isocyanate asthma go away?
If it is noticed early and exposure is stopped completely, symptoms may ease. If exposure continues, the asthma can become permanent. That is why an employee showing symptoms needs to be assessed quickly by the physician.
How often are PFTs carried out on employees working with isocyanates?
There is no fixed frequency in the legislation. The workplace physician may request a baseline test at pre-employment and sets the subsequent interval according to the level of exposure and the employee's complaints.
An exposure-based test plan, pre-employment and periodic examinations, confidential health files and an examination schedule tracked per person.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.