Skin absorption of isocyanates: Laying of floor screeds
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
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.
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
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