People who follow the "five-second rule" may be better off sticking to the phrase, "when in doubt, throw it out".
So says Dr Jorge Parada, medical director of the infection prevention and control programme at Loyola University Health System. Parada cautioned that as soon as something touches an unclean surface, it picks up dirt and bacteria.
"A dropped item is immediately contaminated and can't really be sanitised," said Parada. The amount of bacteria and what type of microbes are involved depend on the object that is dropped and where it falls, he added.
Rinsing off contaminated items with water may not clean them entirely, but it could significantly reduce the amount of bacteria on it, Parada noted.
"Maybe the dropped item only picks up 1 000 bacteria, but typically the inoculum, or amount of bacteria that is needed for most people to actually get infected, is 10 000 bacteria - well, then the odds are that no harm will occur," he said.
Don't lick dropped items 'clean'
That's not the case for items that are "cleaned" by licking them off or putting them in the mouth.
"That is double-dipping," Parada explained. "You are exposing yourself to bacteria and you are adding your own bacteria to that which contaminated the dropped item. No one is spared anything with this move."
There are levels of contamination, Parada noted. More harmful bacteria can make people sicker, more quickly. He added that a potato chip dropped for just a second on a relatively clean table is likely to be less contaminated than one that falls on the floor and stays there for a while. Parada noted that some items will pick up microbes more easily than others.
"In the same time period, a rock candy is less likely to pick up contamination than a slice of cheese," explained Parada, who is also a professor at Loyola's Stritch School of Medicine.
Don't deliberately expose yourself
Although there is some truth to the idea that exposure to some contaminants could help build up a healthy immune system, Parada advised people to avoid putting themselves at unnecessary risk.
"There actually is certain research that supports the importance of being exposed to bacteria at critical times in a child's development," Parada said. "But I believe this development applies to exposures of everyday living. I do not advocate deliberately exposing ourselves to known contaminants. That would probably be a misplaced approach to building up our defences. If you want to be proactive in building up your defences, eat right, exercise, and [get] adequate sleep - and remember to get your vaccines."