Isocyanates in the workplace

Occupational exposure to isocyanates – considered dangerous substances – is restricted in many countries.

by Martin Heels

Isocyanates are a family of highly reactive chemicals, commonly used in the manufacture of polyurethane paints and adhesives, and with applications in many industries across the world.

Polyurethane is a versatile polymer that is often used in the footwear industry in soling materials, foams, coated upper materials, fabric finishes and elastane fibres. There are three commonly used isocyanates (described later), all of which are known to cause harm to human health. Although after manufacture the reacted isocyanate forms a polymeric substance with very few health concerns, the dangers during the manufacture of polyurethanes have resulted in restrictions on isocyanates exposure in the workplace.

Workplace exposure limits

In the UK, the Health and Safety Executive document EH40/2007 defines the maximum ‘Workplace Exposure Limits’ (WEL) for many chemicals with reported health concerns. Isocyanates have one of the lowest maximum exposure levels of any listed chemical: 0.02 milligram per cubic metre of air (mg/m³), over an eight-hour working period. Other regions of the world have similar low exposure levels. In the USA, for example, the Federal Occupational Safety and Health Administration (OSHA) has set a maximum airborne ‘Permissible Exposure Limit’ (PEL) for individual isocyanates of 0.02mg/m³. PELs are usually expressed as an average exposure over a set period of time, in the same way as WELs. German workplace exposure limit stated in TRGS900 AGW is slightly higher, at 0.05mg/m³. Other EU countries vary, with some being as high as 0.11mg/m³.

The dangers of isocyanates

As these substances are highly reactive, symptoms can occur soon after either direct skin contact or isocyanate vapour is inhaled. Isocyanates are powerful irritants to the respiratory tract and mucus membranes around the eyes. These symptoms can occur when exposure levels are within the WELs, and exposure can cause sensitisation which can lead to severe asthma attacks after repeated exposure.

Specific applications

Three of the most commonly used isocyanates are:

• Hexamethylene diisocyanate, (HDI) – used in the manufacture of paints and contained within two-part spray paints
• Toluene diisocyanate, (TDI) – can be present in adhesives or resins used in the footwear industry
• Methylene diphenylene diisocyanate, (MDI) – present in the manufacture of polyurethanes.

Two-part spray paints and two-part adhesives each contain two solutions that are mixed together immediately prior to being used. One solution contains the polyol (the precursor or monomer), with the second solution containing the isocyanate. It is normal practice with these applications and in the manufacture of polyurethanes that the polyol is added in excess, so that there is no unreacted isocyanate in the final product.

Exposure to isocyanates

Preventing workers’ exposure by the use of control measures such as working in closed systems or using sufficient Local Exhaust Ventilation (LEV) should be the principle methods for minimising exposure. In the UK, businesses using isocyanates are obliged to monitor workers’ exposure through regular biological exposure monitoring. Isocyanate air monitoring can also be used.

Air monitoring

SATRA can supply monitors to quantify the concentration of isocyanates in the air. These are designed to be worn close to the operative’s breathing zone. Any isocyanates present in the air are retained within the device. After a two-hour monitoring period, the device is returned to the laboratory for analysis. Further control measures may be necessary, depending on the results of the monitoring.

Publishing Data

This article was originally published on page 32 of the November 2011 issue of SATRA Bulletin.

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