As for me, I don't care about the variables. When I walk in "schmutz" on the road, I think, "Ew, that's going into my house". Somehow, whenever I drop something on the floor, that visual of "What's on your shoes?" makes me wretch, well, almost, and I throw it away...most of the time.
A study out of Aston University in England seems to confirm the 5-second rule which says that it's safe to eat food you've dropped. Well, mostly.
In the article I read, Larry Greenemeier wrote, "Food retrieved just a few seconds after being dropped is less likely to contain bacteria than if it is left for longer periods of time". Makes sense. I envision that if the food gets picked up as soon as possible, the bacteria don't have time to run over and hop on. [Are you smiling?]
If the food you pick up off the floor, the sofa, whatever has pet hair, dust bunnies or anything else that you wouldn't ordinarily eat except for the fact that it's attached to a piece of food you'd like to eat, toss it. There's a "yuck" factor that you just can't live with.
You know the risks. Balance them against the rewards and conduct yourself accordingly.
Once again, one of the mysteries of life has been solved.
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