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EC number: 701-124-4 | CAS number: -
For the risk assessment of MDI-TPG (4,4'-Methylenediphenyl diisocyanate, oligomeric reaction products with 2,4'-diisocyanato-diphenylmethane and [(methylethylene)bis(oxy)]dipropanol) the DNELs derived for 4,4'-MDI (MDI; CAS No. 101-68-8) and polymeric MDI (pMDI; CAS No. 9016-87-9) were used. This decision was based on the fact that MDI-TPG contains ca. 52 % of the MDI isomers 2,4'- and 4,4'-MDI. The read-across approach from MDI/pMDI to MDI-TPG is justified in the attached document "MDI category justification" below.
In the following the DNEL derivation for MDI is presented :
According to the ECHA Guidance on Information Requirements and Chemical Safety Assessment - Chapter R.8 (Version 2.1, Nov 2012), a national occupational exposure limit (OEL) can be used as a surrogate for a DNEL under certain circumstances (R.8.1.1). APPENDIX R. 8-13 is specifying the derivation of DNELs, when a community/national occupational exposure limit (OEL) is available.
For 4,4´-MDI and pMDI the German MAK Commission established a purely health based OEL (MAK-Value) of 0.05 mg/m3for inhalable aerosol referring to an 8-hour exposure period, that is the basis for the official national OEL in Germany (listed in TRGS 900). This OEL is used as a surrogate DNEL for long-term exposure. A ceiling limit value of 0.1 mg/m3was settled. This ceiling limit is used as a surrogate DNEL for short-term exposure. Since irritation to the respiratory tract is the most sensitive health effect these DNELs apply for local effect, in absence of any systemic toxicity no additional systemic DNELs need to be derived (see below for details).
The justification of these OELs is given in the published 4,4´- MDI/pMDI evaluation of the German MAK Commission (MAK, 2008, 2015).
The German Permanent Senate Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area (MAK Commission) is a group of renowned experts in the field of toxicology and occupational medicine from academia, but also from regulatory bodies and industry. The MAK Commission is independently organized within the German Research Association (DFG) and has a close liaison to the European Commission's Scientific Committee for Occupational Exposure Limits (SCOEL). Amongst other duties, the groupproposes maximum workplace concentrations (MAK values -maximum allowable concentration) for volatile chemicals and dusts.Comprehensive justifications for these values and the charters of the group are made publicly available underhttp://onlinelibrary.wiley.com/book/10.1002/9783527694983.
As a basis for the evaluation for MDI, the MAK justification is referring to the EU Risk Assessment (2005) constituting an extensive peer reviewed data assessment and description.
Since the finalization of the MAK documentation for 4,4´-MDI and pMDI in 2008 several new bioassays have been conducted (e.g. toxicokinetics, respiratory sensitization) which were validated in the hazard assessment of this registration dossier and set into perspective with respect to their relevance for the existing OEL. These assays support the existing MAK justification e.g. by substantiating the protective character of the OEL for a MDI specific chemical respiratory allergy.
Character of the general effect
In the evaluation of the German MAK Commission the justification of the OEL for 4,4´- MDI/pMDI is established based on the isocyanate (NCO) group which is common to the monomeric, oligomeric and the polymeric MDIs. This NCO group is highly reactive (see toxicokinetics and category justification for details). Due to this high reactivity of the functional NCO group towards nucleophilic biomolecules the primary health effect of MDI is irritation at the point of contact, whichcan be demonstrated by the numerous acute, subacute and chronic bioassays, and sensitization.
In this DNEL derivation, a special emphasis is on the presentation of the mode of action for the inhalation route of exposure since it is the most relevant route of exposure for workers and oral exposure cannot be anticipated in any (worker or consumer) scenario.
After inhalation exposure, MDI reacts with nucleophilic low and higher-molecular components of the liquid films that cover the airways, glutathione (GSH) represents the most important nucleophile in quantitative terms. The low-molecular adducts or conjugates of MDI are absorbed and direct transcarbamoylation results in plasma protein adducts (albumin, hemoglobin). All observed health effects resulting from exposure to respirable aerosols in acute, subchronic or chronic bioassays and human studies can be allocated to primary alveolar reactivity (respiratory irritation and/or sensitization). No systemic effect other than secondary to primary irritation has been described.
The acute oral and dermal toxicity data indicates very low toxicity in absence of any clinical signs. The key study acute for oral toxicity resulted in an LD50>2000mg 4,4´-MDI/kg bw (Bomhard, 1990), the key study for acute dermal toxicity in an LD50 >10000 mg pMDI/kg bw (Wazeter, 1964a). In conclusion no acute systemic effect was identified which would require the derivation of acute DNELs.
The most sensitive health effect resulting from acute inhalation exposure to respirable aerosols of MDI is irritation to predominately the bronchio-alveolar part of the respiratory tract.
The key guideline acute inhalation study (OECD 403) was performed in male and female rats at five exposure concentrations of highly respirable aerosols of 4,4´-MDI (Pauluhn, 2008).The combined LC50 for both sexes was calculated as 431 mg/m3. All observed clinical signs were indicative for a strong irritation of the respiratory tract.
Determination of protein and LDH in the BAL are known to be especially sensitive parameters for alveolar irritation. A detailed analysis on the early events of acute alveolar irritation was performed following an acute 6h exposure to highly respirable pMDI aerosols. An increased overall concentration of proteins as the earliest indication of a possible irritation of the alveoli in rats was observed starting at 0.7 mg/m3(Pauluhn, 2000, 2004). Based on the rapid reversibility of the increased protein exudation, these effects are attributed to a temporary surfactant destabilization, and are considered as a homeostatic reaction and not to be adverse (NOAEC 0.7 mg/m3). Relevant changes to the bronchoalveolar lavage (BAL) are not observed until higher acute exposures of 2.4 mg pMDI/m3 and clinical symptoms of respiratory tract irritation are not observed until concentrations of 8 mg pMDI/m3.
Acute irritating effects well correlate with the associated subchronic and chronic endpoints (e.g. cell proliferation, see repeated dose toxicity). Therefore, there are no indications of cumulative or over additive effects, pointing towards a concentration rather than a dose dependent effect of MDI on the respiratory tract. As for the oral and dermal routes of exposure no acute systemic effect was identified which would require the derivation of an acute DNEL.
Skin irritation was observed in a study performed in accordance to OECD Guideline 404 with MDI (mixed isomers) in rats (Märtins, 1991) which is supported by human occupational case reports (NIOSH, 1994).
An eye irritating potential of pMDI needs to be anticipated from human occupational case reports (NIOSH, 1994), even though no irritation with MDI mixed isomers (Märtins, 1991) or only slight irritation with 4,4´-MDI at the most (Duprat et al., 1976) was observed in rat assays according to OECD Guideline 405.
In absence of an appropriate dose descriptor for skin and eye irritation from the available experimental data a no effect level or a dose response correlation cannot be established and in accordance to ECHA Guidance on Information Requirements and Chemical Safety Assessment, Part E (Version 2.0, Nov 2012) a qualitative risk assessment for irritation is indicated.
In acute and chronic animal inhalation bioassays MDI aerosols were demonstrated to be irritating to the respiratory tract. Effects were predominantly observed in the pulmonary region (see acute and repeated inhalation toxicity for more detailed description and quantification of observations).
Based on animal and human data MDI is a skin sensitizer. In consideration of human case reports and epidemiological studies the skin sensitizing potential of MDI cannot be regarded as strong. Though animal assays clearly indicate that MDI related skin sensitization is a threshold effect, induction and elicitation thresholds have been shown to not adequately correlate with human data (see skin sensitization). In absence of a reliable dose descriptor for skin sensitization a qualitative risk assessment in accordance to ECHA Guidance on Information Requirements and Chemical Safety Assessment, Part E (Version 2.0, Nov 2012) is indicated.
MDI is a respiratory sensitizer based on human evidence. It is known, that a specific sensitization of the respiratory tract may be induced by the inhalation or dermal route. For diisocyanates a potential relevance of dermal contact for the induction can be derived from animal experiments, although this has not yet been conclusively demonstrated based on human occupational case reports. A detailed presentation of the etiology of the diisocyanate specific respiratory hypersensitivity in humans is included in the justification of the MAK value: in principle exposure of humans to MDI aerosols can cause specific or nonspecific hyperreactivity of the airways. For the specific reactions, asthma (early type, late-phase reaction or dual type) is clearly more frequent as exogenic allergic alveolitis. Bronchial asthma is a known clinical picture triggered by diisocyanates such as MDI.
In general, current mechanistic understanding of allergic responses such as respiratory allergy and allergic contact dermatitis is such, that it can be assumed that the development of sensitization (induction) and also the elicitation are threshold phenomena. This is acknowledged in the ECHA Guidance on Information Requirements and Chemical Safety Assessment Chapter R.8 (Version 2.1, Nov 2012) where it is indicated that (skin) sensitization is generally regarded as a threshold effect. A common initiating step in the induction ofrespiratory allergy andallergic contact dermatitisis e.g. theactivation ofdendritic cells.Furthermore, the ECHA Guidance Chapter R8 is indicating that “…as for skins sensitization, there is evidence that for respiratory sensitization dose-response relationships exist, although these are frequently less well defined (APPENDIX R. 8-11). This view in the ECHA guidance is in line with an extensive literature study which was performed to evaluate these dose-response relationships and related no-effect levels for sensitization and elicitation in skin and respiratory allergy in general. With respect to the respiratory tract, dose-response relationships and no-effect levels for induction were found in several human as well as animal studies (Arts et al., 2006, see respiratory sensitization for details). A more recent literature study with a specific focus on the sensitization of the respiratory tract confirmed the existence of such thresholds for chemical respiratory allergy in general (Cochrane et al., 2015). Besides an updated evaluation of animal experiments, this conclusion was derived from an extensive review of human studies of occupational asthmafrom various chemical substance classes. It is concluded, that even without a numerically defined threshold, or a complete understanding of the dose-response relationship, operational thresholds can be established to avoid induction of sensitization or elicitation of an allergic response. For diisocyanates in specific, the recent publication by Cochrane et al. (2015) also assessed the database for diisocyanates and concluded that the data suggests that there is a threshold for the acquisition of sensitization and that these threshold concentration are in the range of between 5 and 20 ppb.
This conclusion is confirmed for MDI by the evaluation of large number of human occupational studies in larger cohorts by the MAK commission, concluding that “…if a pMDI concentration of less than 5 ppb (0.05 mg/m3) is maintained, no new cases of asthma attacks or asthmatic complaints were observed…” (e.g. Bernstein et al. 1993; in MAK 2008).
This practical experience indicates that induction and elicitation of an MDI specific chemical respiratory allergy would be covered by the existing OEL. However, rare case reports describe elicitation reactions well below the current OEL in sensitized individuals with symptoms, who were continued to be exposed.Medical surveillance of lung function of exposed workers can effectively prevent the occurrence of severe acute asthma attacks (Tarlo, 2002 and 2005).
ECHA Guidance Chapter R8 further outlines that “…at present there are no validated or widely accepted animal or in vitro test protocols to detect respiratory sensitization or to determine the induction or elicitation thresholds.”
In recent years, an asthma model was developed in the BN rat in which dose response correlations and thresholds can be effectively identified. This model was validated with TMA and volatile (HDI and TDI) and non-volatile (MDI) diisocyanates (see respiratory sensitization for details). Asthmagenic responses were demonstrated by means of lung inflammation identified in bronchoalveolar lavage fluid (BALF) and respiratory responses using whole body plethysmography. Both dermal and inhalation routes apply for induction and a single (TMA) or repeated mildly irritating challenges (diisocyanates) are required for elicitation.
A no-effect level forinductionwith MDI was determined at about 5000 mg/m3.mins (i.e. 5 x 10-min/day to 97.1mg/m3and 5 x 360-min/day to 2.9 mg/m3), challenging at 41 mg/m3(Pauluhn and Poole, 2011).
For determining dose response correlations and thresholds forelicitationthe last elicitation challenge is typically performed as group-wise dose escalation (5, 15, 40 mg/m3MDI). A no effect level of 5 mg/m3could be defined for the elicitation (Pauluhn, 2008).
Summarized, for MDI the BN rat asthma model demonstrates the existence of thresholds for the induction by both the dermal and inhalation routes, and a threshold for elicitation of respiratory sensitization following induction and subsequent multiple challenges. The derived elicitation threshold C×t appears to be plausible relative to human evidence. The close association of C×t products triggering an elicitation response in asthmatic rats with the acute pulmonary irritation threshold C×t is intriguing and supports the view that for this class of chemicals portal of entry related allergic responses appear to be linked with pulmonary and/ or lower airway irritation. Accordingly, high concentrations delivered to the respiratory tract during short exposure periods appear to bear a higher sensitizing potency than equal C×t products during longer exposure periods (Pauluhn and Poole, 2011). Pauluhn (2008) examined if the results from this animal model could be applied to the human workplace experience and concluded that “Taking into account the current occupational exposure level of MDI (0.05 mg/m3), the conclusions derived from this bioassay of respiratory irritation and allergy are not at variance with existing human evidence and current occupational workplace standards.”
This covers both the elicitation and induction phase since in accordance to ECHA Guidance Chapter R.8 “…the dose required to induce sensitization in a non-sensitized subject is usually greater than that required to elicit a reaction in a previously sensitized subject”.
Repeated dose toxicity / carcinogenicity
Chronic studies have demonstrated that for repeated inhalation exposure the toxicity of MDI and pMDI is determined by the local irritating effects on the bronchioalveolar region of the respiratory tract. For chronic daily 6-hour exposure (5 days/week) of rats to respirable aerosols, the first indications of irritating effects, e.g. histological effects, are observed from 1 mg pMDI/m3. In comparison to subchronic studies a longer duration of the exposure did not result in a reduction of the effect concentration (NOAEC 0.2 mg pMDI/m3; Reuzel et al., 1990). Concentrations of 6 mg/m3resulted in chronic-inflammatory changes with an increased incidence of bronchoalveolar adenoma and one carcinoma, mainly in the lower respiratory tract. During a long-term inhalation experiment with MDI aerosol with a daily 17-hour exposure (7 days/week), a pulmonary adenoma developed in addition to chronic-inflammatory changes to the respiratory tract at 2 mg/m3(Hoymann et al. 1997). These findings were assessed as typical subsequent reactions of rat’s lungs to a direct irritating effect on the alveoli and the chronic inflammation (regenerative cell proliferation) associated with this. Fibrotic effects may occur secondary to chronic plasma exudation in the alveolar region. Similar findings have also been described for substances with similar effect profiles (alveolar irritation) (Pauluhn et al. 2007). There are no indications of systemic toxicity.
According to the MAK justification human case studies are of relatively low quality (e.g due to co-exposure to toluene diisocyanate and inaccurate exposure specifications). Impairment of the lung function due to long-term exposure was observed with pMDI up to 87 ppb (0.9 mg/m3), whilst for a predominantly observed maximum concentration of 20 ppb (0.2 mg/m3) no significant changes in the lung spirometry were found. However, a few people were found to have airway-related complaints although airway-related complaints were not significantly more frequent for exposures below 10 ppb pMDI (0.1 mg/m3).
For MDI, no repeated dose dermal toxicity studies are available. In a dermal penetration study of MDI in rats only 0.9% of the applied radioactivity was absorbed at the most (Leibold et al.,1999) and systemic effects were not identified in repeated dose inhalation toxicity studies. Therefore is can be anticipated that as for inhalation exposure the local irritative effect and the sensitizing potential is the most sensitive effect for repeated dermal contact to MDI. These effects are covered by the qualitative risk assessment preventing from dermal contact by the appropriate risk management measures.
Genotoxicity and Carcinogenicity
From the present valid in vitro data on genotoxicity no concern can be derived for MDI. A valid micronucleus test in vivo after inhalation exposure to lung-irritating concentrations of MDI had a negative result (Pauluhn, 2001). After chronic MDI exposure via inhalation, no DNA binding was observed in the lungs as the target organ for MDI toxicity (Vock et al. 1996). In absence of formation of free MDA in the lung (see toxicokinetics) a non-genotoxic mechanism based on the local pulmonary irritating effect of MDI can be demonstrated for the formation of lung tumours in rodent studies (see above). This is sufficiently covered by the DNEL derived for local effects in the respiratory tract (see repeated dose toxicity).
In the absence of maternal toxicity identified by respiratory tract irritation no embryotoxic effects occurred in rat developmental toxicity studies (NOAEC developmental toxicity 3 mg/m3MDI, 4 mg/m3pMDI). No systemic toxicity to reproductive organs was identified in subchronic and chronic toxicity studies.
No fertility study including functional parameters is available. But due to the strong irritating properties and the absence of any systemic toxicity in repeated dose studies, there are no exposures to be expected at which there is a realistic possibility of toxicity to fertility.
Conclusion for workers
The primary health effect of MDI and pMDI is irritation at the point of contact and sensitization. In rats effect levels for lower respiratory tract irritation from short-term inhalation studies with corresponding changes in the bronchoalveolar lavage and cell proliferation of the bronchoalveolar epithelial cells (NOAEC 0.7 mg/m3, LOAEC 1 mg/m3) correlate with histopathological findings from studies with chronic lifetime exposure (NOAEC 0.2 mg/m3, LOAEC 1 mg/m3). As shown by this comparison of the acute and long term thresholds the effect of MDI and pMDI on the respiratory tract mainly depends on the concentration level, which is in line with the direct local irritation mechanism of toxicity.
Airway-related complaints in workers were not significantly more frequent for exposures below 10 ppb pMDI (0.1 mg/m3). For the development of a specific hypersensitivity of the airways, exposures of over 0.2 mg/m3or intensive skin contact are important.This is why higher peak concentrations should be avoided.
Based on this human experience the MAK value was set to 0.05 mg/m3(MAK 1992). Higher peak concentrations should be avoided to momentary values of 0.1 mg/m3.
Since both MDI and pMDI have equal amount of reactive NCO groups and have only minor differences between the biological effects of both substances, the MAK value of 0.05 mg/m3now also applies for both MDI and pMDI, representing the official legally binding OEL (TRGS 900, 2006).
All cited studies are reported in the respective end points.
For a detailed description of the data see worker DNELs or the respective End Point Summaries.
Conclusion for general population
The primary health effect of MDI and pMDI is irritation and sensitization at the point of contact.
No reliable dose descriptors can be derived for dermal and ocular irritation and for sensitization via the skin. Therefore a qualitative approach is indicated to appropriately minimize the risk for exposure.
For exposure to respirable aerosols to rats, effect levels for lower respiratory tract irritation from short-term inhalation studies with corresponding changes in the bronchoalveolar lavage and cell proliferation of the bronchoalveolar epithelial cells correlate with histopathological findings from studies with chronic lifetime exposure. As shown by this comparison of the acute and long term thresholds the effect of MDI and pMDI on the respiratory tract mainly depends on the concentration level, which is in line with the direct local irritation mechanism of toxicity.
For MDI, airway-related complaints in workers were not significantly more frequent for exposures below 10 ppb pMDI (0.1 mg/m3). For the development of a specific hypersensitivity of the airways, exposures of over 0.2 mg/m3 or intensive skin contact are important. This is why higher peak concentrations should be avoided.
Based on this human experience the MAK value was set to 0.05 mg/m3 (MAK 1992). Higher peak concentrations should be avoided to momentary values of 0.1 mg/m3.
Extrapolation to the general population should take into account the potential higher susceptibility of sensitive individuals towards respiratory irritation as the most sensitive health effectas well as the different exposure situation. For differences in susceptibility between workers compared to the general population ECHA guidance Chapter R.8 is proposing to apply the quotient of 2 (Table R.8-6).
As mentioned earlier mechanistic studies suggest a close association of C×t products triggering asthmatic responses with the acute pulmonary irritation threshold C×t. This observation supports the view that for this class of chemicals portal of entry related allergic responses appear to be linked with pulmonary and/ or lower airway irritation. The DNELs for local effects for the general population would then sufficiently cover decrements in lung function due to respiratory sensitization of the general population(see respiratory sensitization for details).
Based on the MAK value for workers and a factor of 2 to extrapolate to the general population, the DNEL for local effects is therefore derived to be 0.025 mg/m3 for long term exposure and 0.05 mg/m3 for short term exposure, which would sufficiently cover respiratory sensitization.
Since both MDI and pMDI have equal amount of reactive NCO groups and have only minor differences between the biological effects of both substances, these DNELs applies for both MDI and pMDI.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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