Isocyanates are chemicals widely used in everyday materials like foams, paints, adhesives, and coatings. Used since the 1950s, they are common in many workplaces. Understanding how long these compounds or their breakdown products remain in the body is important for assessing potential health impacts.
How Isocyanates Enter the Body
Isocyanates can enter the body through primary routes, with inhalation being the most common. In occupational settings, airborne particles, mists, vapors, or dusts are encountered. Activities like spray painting or applying foam insulation release these substances, leading to respiratory exposure. Dermal absorption is another route, occurring through skin contact with liquid forms or contaminated surfaces. Skin absorption can significantly contribute to the total body burden. While less common, ingestion is also a possible route, such as accidental hand-to-mouth transfer.
The Body’s Process of Elimination
Once inside the body, isocyanates are highly reactive compounds that undergo rapid transformation. They readily react with biological molecules like proteins and water. This reaction converts the parent isocyanate into diamines like toluene diamine (TDA) from TDI, methylene dianiline (MDA) from MDI, and hexamethylene diamine (HDA) from HDI. These diamines are then conjugated, preparing them for excretion.
The original isocyanate compound is very short-lived due to its high reactivity. The resulting metabolites are primarily eliminated through urine. Some fecal excretion also occurs, particularly for MDI and TDI metabolites, suggesting involvement of the biliary system.
The duration these substances “stay in the body” largely refers to these metabolites. Isocyanate-derived diamines in urine generally have short elimination half-lives, typically 2 to 5 hours for HDI, TDI, and IPDI metabolites. Urine samples collected at the end of a shift predominantly reflect exposure from that day, usually within the previous 10 to 15 hours.
However, MDI metabolites (MDA) can exhibit a longer half-life in urine, potentially exceeding 50 hours, especially following repeated exposures or significant skin contact. This longer half-life means previous days’ exposures can influence current urinary test results for MDI.
Beyond these urinary metabolites, isocyanates can also form more stable protein adducts, such as with albumin, which have much longer half-lives of approximately 20 to 25 days. These adducts indicate integrated exposure over extended periods. Hemoglobin adducts have an even longer lifespan, reflecting exposure over up to 120 days.
Factors Affecting Retention Time
Several factors influence how long isocyanate metabolites persist in the body. The specific isocyanate type plays a role, as different compounds like MDI, TDI, and HDI have varying reactivities and metabolic pathways. The level and duration of exposure also impact retention time; higher or more prolonged exposure results in greater metabolite accumulation, taking longer to clear.
Individual differences also contribute to elimination efficiency. Factors like age, overall health, liver and kidney function, and metabolic rate influence how quickly these substances are processed and removed. While fundamental metabolic processes are similar regardless of entry route, the initial absorption rate can vary. Significant dermal absorption, particularly with MDI, can lead to longer metabolite retention.
Health Implications of Isocyanate Exposure
The presence of isocyanates and their metabolites is a concern due to their potential health effects. Sensitization is a significant health outcome, where the body develops an allergic reaction. Isocyanates are potent respiratory and skin sensitizers.
Once sensitized, even very low levels of future exposure can trigger severe allergic reactions. These reactions often manifest as occupational asthma (breathing difficulties) or allergic contact dermatitis (skin rashes). Sensitization can develop over time, appearing after weeks, months, or years of exposure.
Acute irritant effects are also common. Isocyanates can cause immediate irritation to the eyes, nose, throat, skin, and respiratory tract. Symptoms include chest tightness, coughing, shortness of breath, and skin reactions like rashes or blistering. Prolonged or chronic exposure can lead to persistent respiratory problems, including reduced lung function. In rare instances, chronic exposure can result in hypersensitivity pneumonitis, a serious lung condition, and reports of liver and kidney dysfunction.