Try this simple experiment with your surface disinfectant. Spray or wipe a normal amount on a surface, then step back without wiping and time how long it takes for the liquid to air-dry. Probably a little more than a minute. Now scan the label and note any kill claims that exceed the evaporation time. Yes, it's likely to be a long list. Does this mean that your surface disinfectant isn't effective against organisms that have longer kill claims than the time it took for your disinfectant to evaporate?

No, says the CDC's "Guidelines for Disinfection and Sterilization in Healthcare Facilities 2008," which reads: "Most EPA-registered disinfectants have a 10-minute label claim. However, multiple investigators have demonstrated the effectiveness of these disinfectants against vegetative bacteria (Listeria, E. coli, salmonella, VRE and MRSA), yeasts, MTB and viruses at exposure times of 30 to 60 seconds."

"From a practical standpoint, using a one-minute dry time makes life so much easier. In 30 to 60 seconds, you can pretty much kill everything you have to deal with in a real-life setting," says Ann Marie Pettis, RN, BSN, CIC, director of infection prevention at the University of Rochester Medical Center in Rochester, N.Y. "Staff are not going to sit there and time how long the product takes to evaporate. You have to default to what makes sense, which is letting something air-dry."

But, by the same token, don't just spray it on, wipe it off and presume all organisms are dead. "Nobody's got a 15-second contact time," says Frederick T. Smith of SciReg, science and regulatory consultants based in Prince William, Va.

Let the product air-dry before you wipe it off, says Ms. Pettis. "The longer it takes the disinfectant to air-dry, the greater your chances of killing whatever you want to kill. You don't want to wipe it off and cut short the exposure time," she says.

Most product labels have multiple contact times for multiple organisms. The contact times might range from 30 seconds for HIV to 10 minutes for tuberculosis, says Mr. Smith. "A lot of these labels emphasize that the product works in 30 seconds, which it does — against one organism," he says. "Nobody knows what bugs are on the surface of what you're tying to disinfect. You have to assume that you're going to need the longest contact time on the label to kill the organisms listed on the label."

Back to the same old problem: The exposure times on the product's label and the evaporation times rarely mesh. "You might as well use water," says one manufacturer. "Why buy a disinfectant if it evaporates before it can effectively kill the organism?"

William A. Rutala, PhD, MPH, professor of the department of infectious diseases at the University of North Carolina at Chapel Hill and a foremost authority on surface disinfection, comments on the disconnect between label instructions and what studies show:

• Multiple scientific studies have demonstrated the efficacy of hospital disinfectants against pathogens causing healthcare-associated infections with a contact time of at least one minute.
• The only way you can achieve a contact time of 10 minutes is to reapply the surface disinfectant five or six times.
• There are no data that demonstrate improved infection prevention by a 10-minute contact time versus a one-minute contact time.

"Thus," says Dr. Rutala, "we believe the guideline allows us to continue our use of low-level disinfectants for noncritical environmental surfaces and patient care equipment with a one-minute contact time."