Ways to Address Potential Health Effects
EXPOSURE GUIDELINES
Irritation and sensitization are the primary hazards associated with dermal and inhalation exposures to diisocyanates. Exposure limits have been established by various regulatory agencies regarding allowable airborne concentrations of diisocyanates in the work environment. It is important to remember, however, that while these values represent the best current thinking of toxicologists and industrial hygienists, they offer no guarantee of absolute safety. Therefore, personnel who work with diisocyanates (including both TDI and MDI) need to know and understand the hazards associated with their use and follow those procedures designed to minimize the hazards involved. Since exposure guidelines are reviewed regularly by occupational health professionals, and are changed when new information dictates, users of diisocyanates need to keep themselves informed of the most current guidelines and regulations.
To minimize the risk of irritation and/or sensitization, the Occupational Safety and Health Administration (OSHA) has set a Permissible Exposure Limit (PEL) for MDI and TDI as a Ceiling Limit, which is not to be exceeded at any time during the workday. The Ceiling Limit is equivalent to the Maximum Allowable Concentration (MAC) commonly used in certain European countries. In the United States, the law requires compliance with OSHA exposure limits. In addition to the exposure limits established by OSHA, the American Conference of Governmental Industrial Hygienists (ACGIH), a voluntary standards setting organization, adopted a Threshold Limit Value (TLV) for both MDI and TDI as an 8-hour time weighted average (TWA). The TWA is an airborne concentration for a normal 8-hour workday and a 40-hour workweek and represents conditions under which nearly all workers can be exposed without adverse health effect. The ACGIH has also adopted a 15-minute Short Term Exposure Limit (STEL) for TDI. The STEL is defined as a 15-minute TWA exposure that, like the Ceiling Limit, should not be exceeded at any time during the workday, even if the 8-hour TWA is within the TLV. Things to consider regarding the STEL include: (1) exposures at the STEL should not be repeated more than four times per day, and (2) there should be at least 60 minutes between successive exposures at the STEL (See Table 1).
Table 1—Exposure Limits to 2,4-/2,6-Toluene Diisocyanate and 4,4’ Methylenediphenyl Diisocyanate
| OSHA PEL- C | ACGIH TLV - TWA | ACGIH TLV - STEL | |
| MDI | 0.02 ppm (0.214 mg/m3) as a Ceiling Limit |
0.005 ppm (0.051 mg/m3) as an 8-hour TWA |
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| TDI | 0.02 ppm (0.14 mg/m3) as Ceiling Limit |
0.005 ppm (0.036 mg/m3) as an 8-hour TWA |
0.02 ppm (0.14 mg/m3) as a 15-minute TWA |
Some articles in earlier published literature suggest that approximately 5% of persons exposed to diisocyanates develop diisocyanate-related asthma (Ott et al., 2003; Adams, 1975). Ott et al. (2003) states that since the mid-1970s, where 8-hour TDI concentrations have been maintained below 5 ppb as a TWA, annual occupational asthma incidence rates have been very low, less than 1%. The few new cases of sensitization occurred when short term exposures were above 20 ppb (Ott et al., 2000; Weill et al., 1981). In addition, in a review of the critical data for the TDI OEL, it was stated that “if the exposure concentrations of TDI are kept below 10 to 20 ppb, generally no new cases of TDI asthma are observed” (AGS, 2006). Conversely, peaks of airborne concentrations well above 20 ppb and/or gross dermal contamination seem to play a special role in the sensitization process. Therefore, controlling exposures only by the 8-hr TWA may not prevent exposures capable of producing sensitization. In addition to controlling exposures below the 8-hour TWA guidelines (5 ppb), control exposures below the OSHA Ceiling Limit (20 ppb). Finally, there also is evidence from animal studies suggesting that repeated dermal exposure may play a role in the development of respiratory sensitization. Elicitation of a respiratory response is manifested with subsequent exposure via inhalation. Once a person is sensitized to diisocyanates, inhalation exposure to challenge concentrations as low as 1 ppb, have been shown to precipitate an asthmatic response (Lemiere et al., 2002).
INDUSTRIAL HYGIENE PRACTICES TO MINIMIZE EXPOSURE
Avoiding exposure to diisocyanates through sound industrial hygiene practices is the primary measure for prevention of diisocyanate-related health problems. Good engineering controls, adherence to industrial hygiene practices and training employees to follow the manufacturer’s recommended handling procedures to minimize exposure to diisocyanates are essential for primary prevention. Inform all persons who work with these materials of the potential hazards to health and safety posed by diisocyanates and the procedures designed to minimize such hazards. Properly train all personnel and equip them to respond appropriately in an emergency, to safely clean up spills and leaks, and to protect themselves from direct contact with diisocyanate liquid, or exposure to excessive levels of diisocyanate vapors and aerosols. General experience with diisocyanates has demonstrated that healthy individuals will not be affected by diisocyanate vapor concentrations that do not exceed 0.02 ppm (Henschler et al., 1962). Thus, airborne vapor concentrations are carefully monitored and include correct sampling procedures and equipment and appropriate analytical techniques (5). Personnel also are properly trained in the administration of appropriate first aid. And finally, personnel read and understand current (Material) Safety Data Sheets (SDSs), Technical Data Sheets (TDS), and similar documents before working with diisocyanates.
(5) Eight-hour TWAs may conceal excessive “exposure peaks,” which can potentially induce sensitization when such peaks exceed 0.02 ppm. Thus, a number of regulatory agencies have established Ceiling Limits or Maximum Allowable Concentrations (MAC), which may not be exceeded at any time during the workday.
MEDICAL SURVEILLANCE
Early detection of health effects through medical surveillance is considered secondary prevention, but very important since removal from exposure carries the best prognosis for diisocyanate-related asthma. Medical surveillance consists of pre-placement and periodic medical surveillance examinations. The medical examination includes a respiratory health history, a clinical evaluation, and baseline pulmonary function testing. Contact the manufacturer for additional information.
Careful individual medical assessment by a physician knowledgeable in diisocyanates is advised prior to placement, during periodic evaluations, and for any new or worsening symptoms for workers with a pre-existing history of asthma, which is defined as work exacerbation of pre-existing asthma, or other respiratory disease that may interfere with the safe handling of diisocyanates. The individual assessment takes into account the workplace exposure monitoring as well as the worker’s past and current medical diagnosis. Atopy (allergic diathesis) has not been demonstrated to be a risk factor for the development of diisocyanate-induced asthma (Redlich et al., 2002). However, these individuals may respond to lower levels of a variety of stimuli depending on the severity of the bronchial hyperresponsiveness (Baur et al., 1982; Baur, 1985; Behr et al., 1990). Likewise, individuals with atopy or an inherited allergic tendency (including skin and/or upper respiratory allergies, manifested as hay fever, sinusitis, positive skin tests to common allergens, etc.) or a history of childhood asthma have not been demonstrated to have greater risk for development of diisocyanate-related asthma (Vandenplas et al., 1993). However, due to the difficulty of making an early diagnosis with a similar pre-existing condition, the physician may recommend restricting individuals with symptomatic nonspecific bronchial hyperresponsiveness (i.e., chest tightness, wheezing, shortness of breath) or symptomatic asthma from working with diisocyanates. Individuals with symptoms suggesting any type of bronchial hyperreactivity should consult a physician for an exact diagnosis and counseling. And finally, individuals with specific diisocyanate bronchial sensitization are restricted from any workplace contact with or exposure to diisocyanates.