Which factor commonly reduces the effectiveness of many disinfectants?

• A. Surfaces are dry.
• B. Blood and protein presence.
• C. High ambient humidity always improves efficacy.
• D. The disinfectant is left on the surface for an hour.

Answer: (B)

The key idea is that organic soil on a surface, such as blood or protein, most often reduces how well many disinfectants work. Organic material can react with or consume the active ingredients in disinfectants (for example, oxidizers like bleach are used up by proteins and other soils), leaving less disinfectant available to kill microbes. It can also form a protective layer that shields organisms from contact, making disinfection less effective. Because of this, cleaning away organic matter before applying a disinfectant is essential to achieve proper microbial kill.

Other options aren’t as reliably disruptive. Surfaces being dry can hinder some disinfectants that require moisture to wet and contact microbes, but the impact varies by product. High humidity doesn’t universally improve efficacy for all disinfectants, and letting a surface sit for an hour generally increases, not decreases, contact time and effectiveness.