Skin absorption of isocyanates: Laying of floor screeds

The Challenge

A company laying a polyurethane floor was using a resin mixture containing a methylene diphenyl diisocyanate (MDI)-based hardener. Isocyanates are respiratory and skin sensitisers. They have been identified by HSE as a major cause of asthma and, under the COSHH regulations, there is a responsibility on duty holders to reduce exposure to such substances as far as is reasonably practicable. For these reasons, the company approached HSE for help in understanding what exposure levels the workers were subjected to, and what measures might be necessary to control the risk.

The process of laying the floor involved one worker mixing the resin, another carrying the resin to the application area and two workers spreading the resin with trowels and smoothing with rollers. The two floor spreaders were observed by occupational hygienists from HSE to be wearing short-sleeved tops and general-purpose gloves whereas the other two workers wore long-sleeved overalls.

The Solution

Personal air samples were taken for all the workers and analysed by our scientists. Inhalation exposure, measured as 8hr Time Weighted Averages, was low with all results well below the Workplace Exposure Limit. Results also agreed with static samples indicating no significant aerosol exposure. But MDI was detectable on glove samples from the worker mixing the resin and from one of the floor spreaders.

Pre- and post-shift urine samples were collected over 4 days from all workers and analysed by HSE's biological monitoring team for methylene dianiline (MDA), a biomarker for MDI exposure.

All samples had detectable MDA levels which were higher pre-shift than post-shift, indicating a delayed absorption route which can be explained by dermal absorption. The levels of MDA measured in urine were higher than those expected from inhalation exposure alone at the air concentrations measured.

Statistical analysis also showed a significant correlation between MDI levels measured on gloves and next-day pre-shift urinary MDA levels.

Biological monitoring work showed that dermal absorption of isocyanates is possible and results can exceed the UK biological monitoring guidance value of 1 µmol/mol creatinine.

The Outcome

Evidence for significant dermal absorption of isocyanates was demonstrated and appropriate precautions when handling resins containing isocyanates were subsequently taken by the company involved. By assessing the risk in the workplace and observing worker habits, practical and effective advice could be given to prevent harm.

For more information about how we could do something similar for you, email us at:  registration.sample@hse.gov.uk