Decontaminating refers to the process of removing hazardous substances or contaminants from an object or environment. The antonym of decontaminating, therefore, would be 'contaminating' - the act of introducing harmful contaminants to an object or environment. A related antonym would be 'polluting,' which refers to the introduction of substances that are harmful to the environment. Other antonyms could include 'poisoning' or 'infecting,' which involve the introduction of harmful substances or pathogens. Ultimately, the antonyms for decontaminating refer to actions that pollute, harm, or compromise the safety of an object or environment.