If you live in a dry and hot environment, such as Scottsdale, Arizona, then you don't need anti-microbial protection for your deck cleaner. Anti-microbial formulations kill mold and mildew growth. Hot and arid climates are inhospitable environments for mold and mildew, and we doubt you'll have a problem. However, if you live near the ocean or a lake, where humidity levels are high, you're going to need an anti-microbial formulation.