The nose knows.
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There’s just something about the smell of line-dried laundry.

Is it just nostalgia—say, for grandma’s house or beach towels in summer—that gets our olfactory glands going, or is there actual science behind the fresh smell we love so much?

One group of environmental chemists endeavored to find out. They examined line-dried towels at the molecular level and shared their findings in a paper published this year in Environmental Chemistry.

The research was led by Silvia Pugliese while she was a master’s student at the University of Copenhagen. She told The New York Times that her mother line-dried laundry when she was a child, and she still does it whenever she can.

“The fresh smell reminds me of home,” Pugliese explained.

With a set of cotton towels and ultra-purified water, Pugliese, two of her lab mates, and their advisor took over two areas of the university’s chemistry building: an empty office and a small, fifth-floor balcony.

Each towel got washed three times in the water, and then hung out to dry. Towels were hung either inside the office, on the balcony under a plastic shade, or on the balcony in the sun. After each towel was finished drying, the researchers sealed it in a bag for 15 hours while they sampled the chemical compounds it released into the air around it.

They discovered that line-drying produced a number of aldehydes and ketones, carbon compounds found in plants and perfumes. These weren’t present in the towels dried inside.

"Many of these compounds have smells that are subjectively found to be pleasant,” researchers wrote.

The difference between the drying techniques? Scientists theorize that it has to do with the sun. When exposed to ultraviolet light, certain molecules “get excited” and form highly reactive compounds called radicals, Pugliese told the Times. Those radicals then join with other nearby molecules, which can lead to the creation of aldehydes and ketones like pentanal, which is found in cardamom, and octanal, which produces citrusy aromas.

Mmmmm.

It’s likely that similar processes occur on other natural outdoor surfaces, like soil and grass—part of the reason that sun after a rainstorm creates an intoxicating smell.

Now that’s our kind of science!