Chemistry of Diisocyanates

Q: What are diisocyanates?

A: Diisocyanates are a family of chemical intermediates used to make polyurethane products which are relied upon by many businesses and consumers throughout the world. While diisocyanates are used in many applications, they are particularly important to the transportation and construction industries. Diisocyanates have been used since the late 1940s and their safety and environmental impact have been well studied.

Q: What is TDI and why is it important?

A: Toluene diisocyanate (TDI) is a chemical used in the production of polyurethanes, primarily for flexible foam applications, including furniture, bedding and carpet underlay, as well as packaging applications. TDI is also used in the manufacture of coatings, sealants, adhesives and elastomers. In transportation applications, TDI is used to produce lighter automobile parts, saving weight, which leads to improvements in fuel efficiency and thus energy conservation.

Q: What is MDI and why is it important?

A: Methylenediphenyl diisocyanate (MDI) is a chemical used in the production of polyurethanes. MDI is used primarily in the industrial production of rigid polyurethane foams such as insulation, other uses include coatings, adhesives, sealants, and elastomers used in paints, glues, weather resistant materials, and footwear. MDI can also be used as a binder for wood and to produce mold cores for the foundry industry. Insulation made with MDI helps consumers save on their heating and cooling costs, conserves energy and can lead to a more sustainable environment. 

Q: What is HDI and why is it important?

A: Hexamethylene diisocyanate (HDI) primarily is used as a chemical intermediate in the manufacture of higher molecular weight polyisocyanates. HDI polyisocyanates are used in paint and surface coatings such as automotive and truck refinishing, industrial, maintenance, and performance coatings. Chemical resistant coatings made with HDI helps military vehicles maintain the durability and resistance needed to withstand harsh combat environments.

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Extensive Regulatory Review

Q: Are diisocyanates adequately regulated?

A: The United States chemical industry complies with all federal, state and local regulations, and evaluates products before they reach the marketplace for health, safety and environmental compliance. Diisocyanates are highly regulated chemicals under the authority of the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA). Federal authorities have set fence-line exposure safety limits for diisocyanates emissions to protect both the community and plant employees. The health benchmarks are based on science and reviewed by government officials to prevent health effects. Companies can face significant civil and criminal penalties for noncompliance.

Q: Are there health effects associated with exposure to diisocyanates?

A: Important workplace exposure limits are established and enforced by OSHA and industry. At the recommended workplace concentrations, generally no short or long-term effects would be expected. However, at levels above recommended workplace exposure limits, diisocyanates can be strongly irritating to eyes, nose, throat, skin and lungs. When inappropriate diisocyanate exposure occurs, respiratory sensitization and asthma are the greatest health concerns. Less frequently, prolonged contact with skin can lead to skin allergy. There is no scientific evidence to support that consumer exposure to diisocyanates are contributing to the increased rate of asthma in the general population, including children. 

Q: How will the polyurethanes and diisocyanate industries respond to the Chemical Action Plans?

A: Among other things, ACC strongly supports programs to provide guidance on safe use and handling of diisocyanates products. The ACC Center for the Polyurethanes Industry has a full library of product stewardship documents, videos and training modules to address concerns about polyurethane raw materials related to environmental, health and safety, distribution, use, emissions, and waste issues. Most of these materials are available free at www.polyurethane.org, www.spraypolyurethane.org, and www.spraytruckbedliner.com. We also support research programs so we can better understand key issues such as emissions from products as they are curing; this in turn helps us develop refined guidance. We will continue to work on product stewardship efforts for all diisocyanate products.

Q: Have the polyurethanes and diisocyanate industries been working on any of the issues EPA mentions in the TDI and MDI Chemical Action Plans?

A: Yes. In fact, the MDI action plan references an industry-hosted website providing guidance on the use of spray polyurethane foam (accessible at http://www.spraypolyurethane.org). It also acknowledges that the “federal agencies are working with the polyurethanes industry to ensure accessible hazard communication, applicator training, and best workplace practices to prevent exposure to [di]isocyanates and other SPF chemicals.” Much other work, including several important industry-sponsored research projects, are well under way to promote the development of refined safe handling guidance.

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Worker Safety

Q: How are workers protected from diisocyanate-induced occupational asthma?

A: The vast majority of diisocyanates-based products are produced and used in an industrial setting. Workers are protected through workplace controls, personal protective equipment, training, and provision of safety information, as well as other workplace practices that keep exposures well below levels of concern. Diisocyanates manufacturers and users have medical surveillance and industrial hygiene programs to monitor for respiratory symptoms. Studies have shown that there has been a reduction over the years in occupational asthma associated with diisocyanates.

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Consumer Safety

Q: How are consumers protected from potential exposures to diisocyanates?

A: The vast majority of diisocyanates manufactured are for industrial use. Overall, consumer exposures to unreacted diisocyanates are expected to be of very low magnitude and frequency. EPA notes that “polyurethane products, such as mattresses, pillows, and bowling balls, are considered completely cured products before they are sold.” EPA also states that “[c]ompletely cured products are fully reacted and therefore are considered to be inert and non-toxic.” Diisocyanates are known to cause respiratory sensitization at concentrations above allowable workplace limits; however exposure to such air concentrations are highly unlikely to occur in consumer products. Consumer products containing uncured diisocyanates generally are accompanied by product safety information (e.g. warning labels) including the characteristics of the chemicals. 

Q: Are diisocyanates contributing to the increased rate of asthma in the general population?

A: There is no scientific evidence to support that diisocyanates are contributing to the increased rate of asthma in the general population, including children. In fact, there is no scientific consensus on the causation for the increase of asthma. A large majority of people with asthma have allergies to airborne substances such as tree, grass, weed pollens, mold, animal dander, dust mites, and cockroach particles. Such allergens can act as triggers for asthmatic attacks. The EPA advises that the most common indoor asthma triggers include secondhand tobacco smoke, dust mites, mold, cockroaches and other pests, household pets, and combustion byproducts.

A recent study conducted by Agency for Toxic Substances and Disease Registry (ATSDR) concluded, “We did not find a scientific connection between respiratory problems and exposure to TDI…Overall, we did not find that people living near the plants that emit TDI have recent or current exposure to TDI at levels of health concern.” EPA under their School Air Monitoring Program recently monitored the outdoor air around several schools for diisocyanates and found that the levels were non-detectable and well below levels of concern.

Q: Are children more susceptible to asthma due to their physiology?

A: The case is often made that children are more susceptible to asthma, and the exacerbation of pre-existing asthma, than adults. However, the physiological differences between children and adults (e.g., breathing rates, lung size) can result in the lungs of children receiving a higher dose of asthmogen at any given air concentration. Thus, the incidence of asthma in children may be more reflective of higher asthmogenic doses rather than an inherently higher susceptibility to asthma.

With regard to diisocyanates, it is becoming increasingly clear that the macromolecular and cellular pathways that are associated with childhood asthma and predominate in early childhood (Th2) are different from those associated with the full manifestation of diisocyanate asthma in adults (Th1). This dichotomy in pathophysiology indicates that children are likely to be less susceptible to diisocyanate-induced asthma than adults.   

The special needs and safety of children is an integral consideration in the establishment of community exposure limits. Protecting children's health and well-being is a fundamental value the diisocyanates industry shares with society. Children live safer, healthier lives thanks in part to the development of diisocyanates products and technologies that improve public health and safety.

Q: Does the Jan et al. 2008 study investigating an MDI-based athletic track application provide misleading information linking MDI exposure and asthma-like symptoms in children?

A: The Jan study has been used recently as a basis for concluding that schoolchildren experienced asthma-like symptoms when exposed to an athletic track application containing MDI. In fact, the reported Reactive Airways Dysfunction Syndrome (RADS)-like symptoms (e.g., cough, wheeze, headache) are likely due to xylene, a known CNS depressant and upper respiratory tract irritant that was used as a solvent for the applied MDI. 

This conclusion is based on several facts. First, a closer inspection of the claim by Jan and coworkers that MDI was detected in the air near the polyurethane track proves this statement to be untrue. There is no evidence in this publication or an earlier one that MDI was detected. Second, no biomarker was detected in the urine of school children purportedly exposed to MDI. Third, the symptoms described by Jan and coworkers are likely due to the inhalation of xylene, the major component in the applied product (0.1% MDI in xylene) that is 1 million-fold more volatile than MDI.

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Questions

Q: Where can I find more information about diisocyanates?

A: More information can be found at the ACC Diisocyanates Panel website or the Center for the Polyurethanes Industry website. For more information, please contact Kathryn St. John at Kathryn_StJohn@AmericanChemistry.com or (202) 249-6513.