Ryan Kelly is amazed by what floats invisibly in the air.

"It's completely mind-blowing," says Kelly, who studies environmental DNA (eDNA) at the University of Washington in Seattle. "We are absolutely surrounded by information in the form of DNA and RNA, at all times."

Scientists have long extracted DNA from water and soil, but they've only just begun to look at the air as a source of genetic information. For the past decade or so, researchers have been learning how to measure airborne DNA, study its abundance, and use it to build a picture of an ecosystem's inhabitants and health. Airborne DNA is being used to monitor individual species and is being tested as a method for detecting invasive species or biological weapons attacks. It is also being tested as a way to judge the success of conservation efforts.

The technique promises to link "all biodiversity, the entire world, with a single test that is really fast and can even be done in the field and analyzed in the cloud," says David Duffy, a researcher specializing in wildlife disease genomics at the University of Florida in St. Augustine.

But much remains to be defined, such as the rate at which DNA decays in the air and the distance it travels. Some of the genetic material extracted from the air comes from humans, and several scientists fear that using this technique for conservation research could inadvertently reveal people's ethnicity or whether a person has a genetic disorder—and even be used to identify individuals.

Clouds of DNA
Scratch your head and you'll release DNA-rich cellular material into the air. There, it will mix with DNA from countless other sources: your own and others' exhalations and shed skin, fragments of hair, feathers, droppings, pollen and spores, and microorganisms such as viruses and microalgae. This DNA, which can include segments of tens of thousands of base pairs, will then drift through the air for perhaps a few days, often clinging to dust particles. It can travel distances ranging from a few meters to several thousand.

Although eDNA is already routinely collected from water, snow, and soil to gather information about biodiversity or to track pollutants or viruses, scientists have not typically monitored airborne sources of DNA beyond pollen and spores—robust packages designed to travel on the breeze.

But in the early 2010s, several ecologists began to wonder whether the air might contain useful traces of DNA beyond those wrapped in such wind-borne bundles. In 2013, biologists Matt Clark of the Natural History Museum in London and Richard Leggett of the Earlham Institute in Norwich, UK, sampled air inside and outside a greenhouse.

Meanwhile, at Texas Tech University in Lubbock, ecologist Matthew Barnes analyzed air samples using techniques developed to collect waterborne electronic DNA and found they were teeming with DNA from leaves and flowers, as well as types of pollen not designed to be carried by the wind. He then realized the potential for understanding entire plant communities using air.

But it was the discovery of tiger DNA near Cambridge, UK, that alerted the wider scientific community to the potential of airborne DNA. Elizabeth Clare, of York University in Toronto, Canada, and Joanne Littlefair, of University College London, wanted to know if they could find animal DNA in the air. They collected samples at a small zoo in Cambridgeshire, UK, reasoning that they would know the origin of any DNA they found, since the exotic animals were confined to the park.

In the lab, the researchers extracted the DNA from the samples, amplified it, and sequenced it. They discovered they could detect tigers 200 meters from their enclosure, as well as many other animals in the zoo, their food—including chickens, horses, and pigs—and wildlife such as hedgehogs, bats, and squirrels. In total, the samples contained DNA from 25 species of mammals and birds, including 17 housed at the zoo. Another study near Copenhagen Zoo, published at the same time, had similar findings.

"Animal DNA in the air has always been there; we just haven't looked for it," says Simon Creer, who studies molecular ecology at Bangor University in the UK.

WILEY