Day-old pizza is God's gift to college students, starving artists and anyone who thought it was a brilliant idea to order that extra-large double-meat at 2 a.m. after coming home from the bar, only to go sleepy-peepy halfway through the first slice.
But while cold pizza is a bona fide breakfast of champions, what about room-temperature pizza? Will you get sick if you throw down a few slices of the pepperoni that sat in a greasy cardboard box next to your bed for the last eight hours?
The official answer — don't risk it. The United States Department of Agriculture (USDA) published some food safety guidelines for students in which it answered this very question. According to the USDA, you should throw away any leftover food that's been sitting out at room temperature for two hours or more, whether or not it contains meat.
The reason is that bad bacteria grow the fastest on foods that are in the "danger zone," temperatures between 40 degrees Fahrenheit and 140 degrees Fahrenheit (4.4 degrees Celsius to 60 Celsius). The bacteria actually double in number every 20 minutes.
Does that mean that every pizza is contaminated with pathogenic bacteria that will explode in number if the pie is left out for more than two hours? Absolutely not. Benjamin Chapman, a food safety specialist at North Carolina State University, told Lifehacker that leftover pizza hasn't made enough people sick to count as a public health risk.
Chapman says it's probably because pizza toppings and crust are generally too dry to be bacteria-friendly environments and that tomato sauce is too acidic. Not all toppings are created equal, though. Pepperoni is dry cured, so it's built to last. But eating old veggie ingredients or moist chunks of chicken is probably pressing your luck.
To get a sense of the general risk level of pizza, check out this public health report from Ontario, Canada. According to a review of global food poisoning databases, pizza has been implicated in a number of foodborne illness outbreaks worldwide, and that includes pizzas of all types (plain cheese, meat, veggie) in both restaurants and in homes.
For some perspective though, that report cited a few hundred individual cases of food poisoning over more than a decade of worldwide pizza-eating. In the U.S. alone, we eat an estimated 3 billion pizzas every year.
So should you finish off those last two pieces of stuffed-crust Hawaiian from last night's poker game? The odds of getting sick are probably similar to the odds of drawing a royal flush. So the real question is, are you feeling lucky?