Hardeners (or activators) contain which chemical group that is toxic to the respiratory system?

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Isocyanates are the chemical group in hardeners that poses respiratory toxicity. When these hardeners are mixed, isocyanate groups can become airborne, and inhaling them is a major hazard. They are well-known respiratory irritants and potent sensitizers; exposure can trigger asthma-like symptoms and long-term breathing problems, even at relatively low levels if protective measures aren’t used. That’s why proper ventilation, appropriate respirators, and minimizing exposure are important. Acrylates and epoxies can cause irritation and sensitization as well, but the strong, well-documented respiratory danger in hardeners comes from the isocyanate group. Alcohols are less associated with respiratory toxicity in this context.