Sep 20, 2018 11:00 AM

Invisible Swarms of Particles Envelop Us All. Come Have a Look

Scientists have demonstrated a promising technique for measuring and mapping your exposome—the flecks of animate and inanimate stuff that surround you at all times.

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For two and a half years, Michael Snyder began each day by strapping a device to his arm the size and shape of a large matchbox. Gray in color with a nozzle extending from its top, the box went with him everywhere, whether he was traveling abroad, hanging holiday decorations at his home in Palo Alto, or biking to his office at Stanford. Its purpose: To capture the various airborne particles he encountered in his daily life.

"For years we've been sequencing people's genomes, testing their blood and urine, and analyzing the microbes in their guts to understand how these things impact human health," says Snyder, a systems biologist and chair of the genetics department at Stanford. "But all of those things have to do with what's inside your body. The one big thing we're missing is: What are you exposed to?"

Paul Sakuma/Stanford School of Medicine

That's precisely the question Snyder's apparatus is meant to answer. It's designed to record an individual's exposure to the world around them (in particular the kind of stuff that can infiltrate their lungs, settle down on their skin, or go forth and multiply on one of their mucous membranes), what researchers have recently taken to calling their exposome. Snyder calls the device a modified air monitor. I call it an exposometer. In any case, its battery-powered pump samples its wearer's immediate surroundings in slow, steady sips as they go about their life. In its filters it traps all manner of miniscule matter: bacteria and viruses; fungi and pollen; insecticides and carcinogens. Liquid or solid, organic or inorganic, if a particle riding the wind wafts by the exposometer's intake valve, it becomes ensnared.

For a study recounted in this week's issue of Cell, Snyder and 14 other research participants donned the devices—some for a week, others for a month, one for a year, and Snyder for 890 days—as they passed through more than 60 distinct locations. Every so often, Snyder and his team collected the exposometers, sequenced the biological material in their traps, and analyzed the abiotic bits with a mass spectrometer. Finally, they translated their tens of billions of readouts into a colossal database of environmental exposures some 40,000 entries strong—the first such catalogue of its kind.

The researchers also analyzed the makeup of individual exposomes. They found them to vary considerably between test subjects, including those who live and work within a relatively small geographic region like the San Francisco Bay Area. In the course of a month, exposometers collected from test subjects living in Palo Alto, Sunnyvale, Redwood City and San Francisco revealed very distinct exposure patterns. The exposome profile of the San Francisco resident, for example, turned up higher rates of so-called sludge bacteria. Sludge as in sewer water, or feces-of-unknown-origin on a sidewalk south of Market Street.

The test subjects' exposomes also hinted at how certain particulates might affect human health. For years, Snyder suffered from allergies in the early spring, which he assumed were associated with pine pollen. Yet his exposome signature suggests his symptoms correlate more strongly with eucalyptus. Similarly, analyses of test subjects' exposomes revealed interesting correlations between certain particulates. Among the most surprising was an apparent inverse relationship between pyridine—an additive, once commonly found in house paints, associated with all manner of nasty side effects— and fungus. Exhibit A: Snyder's own home. "When it was remodeled in 2010, my contractor used pyridine-free paint. "Today, my house is a fungal explosion," he says. "It is what it is. But that could be a big deal to somebody who's severely allergic to black mold." To someone like that, Snyder says, a paint containing pyridine could actually be a net positive.

Here's the thing, though: All of these connections between particulate exposure and health remain largely untested. Snyder is confident that exposome research will bring droves of those relationships to light, and that they'll be very important to human health in the long run. But this current study? "It's mostly descriptive," he says. "The exposome is vast. It's dynamic. Understanding it is going to impact our health in the future, and this study is the first step toward mapping it."

Microbiologist Jack Gilbert, director of the Microbiome Center at the University of Chicago and head of Argonne National Laboratory's microbial ecology division, takes a somewhat harsher tack: "At best, this is an observational study that says: When you move about, you're exposed to different things. But it doesn't really go beyond that," he says. A bolder step might involve tracking test subjects’ immune response in conjunction with their exposomes, by, say, collecting daily blood serum samples from research participants. "Don't get me wrong—I love the work, but I question its impact on our current understanding of people's exposomes."

Gilbert—who reviewed the study for the journal Science, before it was re-submitted to Cell—says that, the way he sees it, the study's biggest contribution is technological, not scientific. It all comes back to the exposometer. Prior exposome research has involved placing stationary air monitors in places people frequent, like houses and subway stations and busy intersections. Snyder's gadget inverts the sampling process by making the monitor simultaneously mobile and highly personal: By attaching it to an individual, you can record every speck of biological and chemical matter that enters their orbit, specifically.

"Having this sensor and showing that it can be used to measure individual exposomes? That's kickass," Gilbert says. "I love it. That's what I need in my own research. So we'll definitely be doing that, and leveraging it going forward."

Snyder, meanwhile, compares his present study to sequencing the human genome for the first time: "What did we learn? Well, we learned there were genes there! It seems obvious, but it was an important milestone because it showed what was possible. It laid out the blueprint. That's what we've done here."

The next step, he says, is to collect more data, under more tightly controlled experimental conditions, from more people. A lot more. As in: Thousands-of-test-subjects more.

But scaling poses another challenge: Cost. The current iteration of the exposometer runs $2,700, pre-modifications, and is apparently quite easy to lose; in the past few years, Snyder, alone, has misplaced four. Which means that, before exposome research can revolutionize personal medicine, someone might have to develop a less expensive monitoring device. Either that, or a version that wearers won't mind being seen with in public. Let's face it: Its big-gray-boxiness makes the exposometer a pretty tough sell, as a fashion piece.