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Formaldehyde, glutaraldehyde, and phenol are considered hazardous primarily because they are toxic to living tissue. These chemical agents can cause a range of harmful effects, including irritation of the skin, eyes, and respiratory system, and, with prolonged exposure, can lead to more severe health issues. For example, formaldehyde has been associated with respiratory distress and carcinogenicity, while glutaraldehyde is often used in high concentrations as a disinfectant, which can result in significant tissue damage upon contact. Phenol, known for its ability to penetrate biological membranes, can cause systemic toxicity and damage to organs when absorbed by living tissues. This inherent toxicity is a key factor in why they are classified as hazardous substances.

While the strong odors of these substances can be concerning and may indicate their presence, odor alone does not determine their hazardous nature. Non-toxicity is contrary to their established effects on living organisms, and while allergic reactions can be associated with these chemicals, the primary and more direct danger comes from their toxicity to living tissue.