What is a sign that a disinfectant is effective?

A sign that a disinfectant is effective is indicated by the presence of an EPA registration number on the label. This registration number signifies that the disinfectant has been evaluated and approved by the Environmental Protection Agency for its efficacy in killing specific pathogens and for meeting safety standards. The EPA registration process involves rigorous testing to ensure that the disinfectant performs as claimed against bacteria, viruses, and fungi.

The other options, while they may seem relevant, do not reliably indicate a disinfectant's effectiveness. A strong odor does not necessarily correlate with potency; some effective disinfectants are odorless or have a mild scent. Foaming action can indicate the presence of surfactants, but it does not directly measure disinfecting capability. Similarly, a color change after exposure to air is not a standard method for assessing disinfectant effectiveness and may vary widely between products without relating to their efficacy.