Do Farts Cause Frost on the Windows? On water vapor, humidity, cold weather, and flatulence

Slug Signorino

In the midst of a below-zero cold spell, my significant other and I were discussing the sources of humidity in our home that resulted in frost on the windows. He said breath. I said farts were also a factor. He said I was crazy. I’m not saying the two contribute equally, but come on. Cecil, how much moisture is in each of these forms of bodily exhalation? —Barbara Becker

OK, so maybe I should have held this question till January rather than answering it in June. But some things just can’t wait. The answer proved more challenging than you might think.

Human flatus is made up primarily of carbon dioxide, oxygen, nitrogen, methane, and hydrogen, with small but often pungent quantities of hydrogen sulfide and other organic compounds in addition. The volume of flatus varies widely based on diet, how much air you swallow, and how efficiently your gut bacteria digest what you eat and produce gas.

However, while we found many studies detailing the chemical composition of flatulence, none gave a value for water vapor. Sometimes this was due to how the flatus was collected: Bubbling the gas into a flask of displacement solution inevitably means altering its moisture content. No matter—we have other resources. I asked Una, my assistant and a professional engineer, to calculate the maximum quantity of water vapor contained in a fart.

She gave me the look she always gives me. I said what I always say: “It’s for science.” How she does that thing with her eyebrows only she and Jack Black know. Nonetheless, she prepared the following analysis:

1. The amount of flatus produced daily can be more than four liters, but typically it’s around 0.4 liters per day.

2. We’ll assume that, given how moist we are inside (more on this below), any expelled gases contain water vapor at close to 100 percent relative humidity. Calculating the water fraction, and multiplying the result times two people, we arrive at a total moisture output of about .04 milliliters per day.

3. That’s not much. Assuming an average-size dwelling, well sealed off from the winter air and heated to 70 degrees Fahrenheit with a starting relative humidity of 35 percent, a day’s total flatus from two people will theoretically raise that humidity to 35.001 percent.

4. But flatulence accounts for only part of the human contribution to ambient water vapor. Our bodies are made up of 50 to 70 percent water, of which 5 to 10 percent is cycled through us every day. A significant amount of water is lost through breathing and “insensible sweating”—that is, the constant low-level perspiration you don’t notice. A typical sedentary adult loses about 300 milliliters of water per day through breath and 1,175 milliliters per day via insensible sweating, or close to a liter and a half all told.

5. Exhaling only through your nose reduces moisture loss by more than 40 percent, but let’s assume our two test subjects are mouth-breathing adults who spend the entire day in their house. The total daily water contribution from their breath and sweat will be about three liters per day—about 75,000 times that produced by flatulence. Assuming the house is completely sealed, this will increase the relative humidity from 35 percent to almost 70 percent. So it’s fair to say that, while flatulence makes no appreciable difference, breath plus sweat contributes significantly to the frost on your windows. (Have you thought about getting double-glazed windows? Your house sounds like an energy sieve.)

6. Pets also contribute to indoor humidity. Cats and dogs don’t sweat as much as we do; most of their water loss other than through excretion comes from breathing and (in dogs) panting. A typical indoor cat might lose 45 milliliters of water per day through respiration and minor sweating, whereas a Labrador retriever might lose 360 milliliters through breathing, panting, and other sweating.

7. Therefore, a cat will raise the average home’s humidity to 35.4 percent—one cat going about its business is a far bigger factor than two people farting. A large dog in that same situation will increase the relative humidity of the house to more than 39 percent.

8. If we assume a household of two people, a dog, and a cat in a sealed home, then the inhabitants’ total contribution to indoor humidity is about 3.4 liters (close to a gallon). This would increase the relative humidity of the house from the baseline 35 percent to more than 75 percent. The contribution of flatus is negligible. I don’t know that you’re crazy, but with respect to farts being a factor in winter frost accumulation, you’re indisputably wrong.

Keep in mind that houses leak a fair amount of moisture, and every time you open the door results in an exchange of air, so the practical impact of all that exhaled water is much reduced. Remember also that I don’t know your partner. If he’s a member of the four-liter club, flatuswise, all bets are off.

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