When celebrating birthdays, blowing out a cake full of candles and singing a public domain song is a time-honored tradition. Yet has it ever occurred to you that this ritual means that the birthday celebrant is just blasting spit over the candles and the top layer of the cake? Well, now it has.

The Journal of Food Research in Canada recently published a study, which we learned about from The Atlantic, that will make every celebration that you’re part of seem just a little more gross. Logically, some saliva must end up on cakes when you blow out candles, but the daughter of Paul Dawson, a professor at Clemson University, wondered how much more.

The answer: It can be a lot, and varies from person to person, but cake germs are not going to kill you. Probably. Maybe skip the cake if someone looks sick?

The study

To find out, students at Clemson spread foil on a Styrofoam base, then spread frosting on the foil. To make sure everyone’s saliva was going and to simulate a party environment, the test subjects ate pizza before proceeding to the fake cakes.

For each of the eleven test subjects, they lit the candles and blew them out, then took samples from the frosted area and grew them in agar.

Wide variation in spit germ levels

On average, blowing out candles on a sample meant fifteen times more bacteria (1,500%) was present. There was a lot of variation from person to person in the amount of saliva they emitted, the bacteria hanging out in their mouths, or both. The biggest difference compared to the control cake was between three times greater (300%) and 140 times greater (14,000%).

Here’s the thing, though: Unless the person celebrating his or her birthday is obviously sick, these mouth bacteria are the same ones that we come in contact with every day. Do you kiss people? Have you stood near a person who coughed or sneezed? Do you venture out in public at all?

If so, you’ve come in contact with the same spit bacteria that people emit when blowing out candles. It’s just that now we all know that they’re there. On the cake.

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