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Isocyanates:

Health hazards
Isocyanates represent a family of compounds each with unique properties and common hazards. Methylene bisphenyl isocyanate (MDI) and Toluene-2,4-diisocyanate (TDI) are perhaps the most commonly encountered isocyanates, though there are many used in surface coatings, adhesives and manufacture of various foams that are ubiquitous world wide. Isocyanates are used in spraying applications due to their low molecular weight. Like most aerosols they can affect the respiratory system and skin. OSHA ran a National Emphasis Program on isocyanates starting in 2013 to make employers and workers more aware of the health risks.

Respiratory
Isocyanates can present respiratory hazards as vapors or aerosols. Exposure, commonly but not exclusively due to spraying can lead to respiratory irritation, occupational asthma and hypersensitivity pneumonitis. Occupational asthma and hypersensitivity pneumonitis are somewhat similar from a clinical symptom standpoint in that they both generate dyspnea and chest tightness but differ in the underlying cause with asthma being mediated by IgE and hypersensitivity pneumonitis being mediated by neutrophils. Hypersensitivity pneumonitis is slower onset and features chronic inflammation that can be seen on imaging of the lungs. Occupational asthma is a worrisome outcome of respiratory sensitization to isocyanates as it can be acutely fatal. Diagnosis of occupational asthma is generally performed using pulmonary function testing (PFT) and performed by pulmonology or occupational medicine physicians. Occupational asthma is much like asthma in that it causes episodic shortness of breath and wheezing. Both the dose and duration of exposure to isocyanates can lead to respiratory sensitization.

Dermal
Dermal exposures can occur via mixing, spraying coatings or applying and spreading coatings manually. Dermal exposures to isocyanates is know to lead to respiratory sensitization. Even when using the right personal protective equipment (PPE), exposures can occur to body areas not completely covered. Isocyanates can also permeate PPE, necessitating frequent changes of both disposable gloves and suits if they become over exposed.

Carcinogenic
Both the US National Toxicology Program (NTP) and International Agency for Research on Cancer (IARC) have evaluated TDI as a potential human carcinogen and Group 2B "possibly carcinogenic to humans". MDI appears to be relatively safer and is unlikely a human carcinogen. The IARC evaluates MDI as Group 3 "not classifiable as to its carcinogenicity in humans."

Flammability
Methyl isocyanate (MIC) is highly flammable. MDI and TDI are much less flammable. The specific flammability hazard is noted on the safety data sheet (SDS) for specific isocyanates.

Hazard controls
Hierarchy of hazard controls

Elimination & substitution
Elimination and substitution seeks to eliminate a hazard directly from use in industrial processes. Elimination if possible also has the possibility of eliminating the need for other controls. If unable to make an elimination substituting a less hazardous isocyanate may also control hazards. Because of the hazards inherent isocyanates there is ongoing research for suitable replacements.

Engineering controls
Engineering controls seek to decrease hazards by creating barriers to hazard exposure. Using the Source-Pathway-Receptor model, an engineering control acts on the pathway to mitigate hazards emanating from the source from reaching the receptor. An automated spraying booth with a separate ventilation system would be an example of engineering controls. Appropriate ventilation is a common engineering control when using isocyanates.

Administrative controls
Administrative controls are policy or training based controls to decrease hazards. A quarterly training session on recognizing symptoms of occupational asthma or proper respirator use would be examples of administrative controls. Administrative controls can be effective in reducing hazards for which personal protective equipment does not exist, for example no eating or smoking in work areas can prevent ingestion of hazardous chemicals. Training is required by OSHA

Personal protective equipment
Personal protective equipment (PPE) is the lowest level of hazard control. For isocyanates commonly used PPE include respirators for inhalation hazards and gloves to minimize absorption of dermal hazards. PPE like respirators are sensitive to fit and require some maintenance periodically. Eye protection is an important component of PPE. Gloves and coveralls are appropriate personal protective equipment for workers. Gloves and protective clothing may be more intuitive but user resistance can arise due to loss of tactile sensation or increased thermal burden. The material and thickness of gloves is an important component of protection.

Occupational exposure limits
Exposure limits can be expressed as ceiling limits, a maximum value, short term exposure limits (STEL) a 15 minute exposure limit or an 8 hour time weighted average limit (TWA).

Exposure assessment
Exposure assessment is the domain of industrial hygienists. OSHA guidelines provide detailed technical guidance on measuring isocyanates via sampling and analytics procedures tailored to specific chemicals. In the case of MDI, sample is via glass fiber filters at standard air flow rates and liquid chromatography.

Occupational health surveillance
Occupational health surveillance primarily the domain of medical professionals. This can included counseling, respirator fit testing, tracking of biologic exposure via biologic exposure indices (BEI) and PFT results. Biologic monitoring levels for isocyanates exist but may not commonly used. One example of a monitoring program by the United States Navy relies on pulmonary function testing and screening questionnaires.

The combination of industrial hygiene and medical surveillance can have a significant affect on the incidence of occupational asthma.

Emergency preparedness
The Bhopal disaster involving release of MIC and resulting in the deaths of thousands of people and affecting hundreds of thousands more. As a result of major industrial incidents like this, public health officials have proposed disaster preparedness programs aimed at assessing hazards, prevention via engineering and coordinated responses. More recently MIC was involved in an explosion at a pesticide manufacturing plant in West Virginia. Emergency management is a complex process of preparation and should be considered in a setting where a release of bulk chemicals may threaten the well-being of the public.

United States of America
The Occupational Safety and Health Administration (OSHA) is the regulatory body covering worker safety. OSHA puts forth permissible exposure limit (PEL) 20 ppb for MDI and detailed technical guidance on exposure assessment.

The National Institutes of Health (NIOSH) is the agency responsible for providing the research and recommendations regarding workplace safety while OSHA is more of an enforcement body. NIOSH is responsible for producing the science which can result in recommended exposure limits (REL) which can be lower than the PEL. OSHA is tasked with enforcement and defending the enforceable limits (PELs). In 1992 when OSHA reduced the PEL for TDI to the NIOSH REL, the PEL reduction was challenged in court and the reduction reversed.

The Environmental Protection Agency (EPA) is also involved in the regulation of isocyanates with regard to the environment and also non-worker persons that might be exposed.

The American Conference of Government Industrial Hygienists (ACCGIH) is an non-government organization that publishes guidance known as Threshold Limit Values (TLV) for chemicals based research as constant work exposure level without ill-effect. The TLV is not an OSHA enforceable value unless the PEL is the same.

European Union
The European Chemicals Agency (ECHA) provides regulatory oversight of chemicals used within the European Union. ECHA has been implementing policy aimed at limiting worker exposure through elimination via lower allowable concentrations in products and mandatory worker training, an administrative control. Within the European Union many nations set their own occupational exposure limits for isocyanates.

International groups
The United Nations via the World Health Organization (WHO) together with the International Labor Organization (ILO) and United Nations Environment Programme (UNEP), collaborate on the International Programme on Chemical Safety (IPCS) to publish summary documents on chemicals. The IPCS published one such document in 2000 summarizing the status of scientific knowledge on MDI.

The IARC evaluates the hazard data on chemicals and assigns a rating on the risk of carcinogenesis. In the case of TDI the final evaluation is possibly carcinogenic to humans (Group 2B). For 4,4′-Methylenediphenyl diisocyante the final evaluation is not classifiable as to its carcinogenicity to humans (Group 3).

The International Isocyanate Institute is an international industry consortium that seeks promote the safe utilization of isocyanates by promulgating best practices.