Once large enough to be mistaken for terrorist bombs, portable air pollution monitors are now being shrunk into smaller and smaller wearable devices that can be easily dispatched for environmental detective work: Is black carbon soot emitted by school buses contributing not just to warming global temperatures, but raising childhood asthma rates, too? These new pocket-size sensors could provide more practical and powerful detection of such potential public health risks.

Environmental health scientists have grown increasingly interested in personal air pollution tracking in recent years. They realize that bringing monitoring down from the rooftops—where devices have breathed cities' concoctions of exhaled pollutants for many years—can help to identify the variability in exposures among people as well as during an individual's day-to-day activities.

Average particulate matter concentrations across regions, for example, rarely reveal the specific air particles people breathe in any given location. "The problem is, not many people live on rooftops. Most people live in floors as you go down, and walk about on city streets, and get around by cars, subways or buses," says Steven Chillrud, an environmental geochemist at Columbia University's Lamont–Doherty Environment Observatory. "So, if you really want to know what people are exposed to, you need to monitor them."

This is especially true of children. "They get exposed to stranger dust in half an hour of playing than you or I would get in an entire month," says James Cowin of the Pacific Northwest National Laboratory in Richland, Wash., recollecting his childhood days chasing after trucks spouting clouds of DDT.

Takin' it to the streets
This fall, dozens of nine- and 10-year-olds will be set loose on the streets and in the schools of New York City with new monitors co-developed by Chillrud, Cowin and their respective teams. Each device features technologies designed to pinpoint not only what children are exposed to, but when, where and for how long.

Chillrud is one of eight grantees currently refining personal airborne pollution sensors as part of the National Institute of Environmental Health Service's (NIEHS) Genes and Environment Initiative—a project aimed at gaining an improved understanding of how genetic factors and environmental exposures influence human disease. "As we were developing the initiative," says David Balshaw of the NIEHS, "we came to the conclusion that we needed to develop the capacity to monitor individual exposures."

The gold standard for such devices had traditionally been monitors housed in burdensome backpacks, weighing as much as three kilograms, or about as much as a newborn baby. "This limited us to enthusiastic teenagers, or concerned adults," Chillrud notes. "You couldn't really do young kids." And even with the most eager, compliance was always an issue: Would study subjects consistently lug them around or would they grow tired and leave them sitting at home or in a locker?

Before long, however, terrorism took over as the sensor's primary obstacle. "My timing was perfect: the Madrid bombing had happened while we were developing the monitor," Chillrud says. "No one seemed phased much. But when the London bombings happened, and it became apparent that our pack was the same size, the police said, 'No way. You need to shrink it to the size of Walkman, or else you are putting your subjects at risk of being shot.'"

Shrinking the sensor
So, that's just what he did with help from Cowin, who leads the hardware and software development side of the project. "The sampler is not cell phone–size yet (the 1991 models, notwithstanding) but it is pretty compact," Cowin notes, adding how much quieter it is compared with the early, humming models.

The work in progress—currently 15 centimeters long, less than 7.5 centimeters wide and weighing about as much as a Walkman—is undergoing validation to ensure that filters only 0.2 centimeter in diameter can really do equivalent work to the old 3.7-centimeter-wide models. The prototype houses six of these pollution nets: three for the collection of black carbon and three for single particles, both prevalent in urban air and suspected hazards for children's health.

Each pair of filters is then designated for one of three key locations frequented by children: home, school and outdoors (or commuting). And the smart sampler automatically knows where it is at all times. If the kid is at home, a Bluetooth beacon informs the sensor and it switches valves accordingly. When the sensor loses the signal, it then must decide between the other two locations. An inputted schedule helps it determine when to switch to school mode. (This is overridden if, for example, a kid is home sick and the Bluetooth signal from there is picked up.) During all other times, a subject's location is determined using global positioning system technology (GPS). "Someday, when GPS gets even better, we can do all the switching based on it," Chillrud predicts. And, as other technologies continue to improve, further shrinking of the sensor is planned, along with real-time black carbon and particulate matter monitoring.

Although the handheld device is easier to wear than a backpack, it is not problem-free. "As all these samplers get smaller and smaller, compliance becomes a big issue," Chillrud explains. "Before the question was, 'Do you want to carry a big backpack?' Now, it is, 'Do you carry it the right way?'" Measurements could be significantly off if the device is hooked to a belt, put in a purse or backpack, or even covered up by a winter coat. "There are lots of places they can go that we don't want them to be," Chillrud says. But with the help of motion sensors and a specially designed vest, they hope to keep them in the right place.

Sensing new policy
If all goes well in the project's validation stage, the sensors will soon be dispatched in the field with 30 asthmatic and 30 nonsufferers, a subset of several hundred kids that have been studied by researchers since they were in the womb—when their pregnant mothers wore the original backpack monitors. These kids will also be keeping symptom diaries and undergoing clinical assessments; the filters recording their exposures will be analyzed in the lab.

If a physical effect is seen, the researchers can go back and track exposures. Together with the airborne offenders that get caught by the filter, tracked locations can also lend valuable clues: "Were they walking along a busy street? If so, it's likely automobile traffic," NIEHS's Balshaw says. "Were they in the house—in the kitchen? Then it could be cooking grease."

If researchers could correlate one to one what people are exposed to and their health effects, Cowin suggests, "then we could better understand the risks and add that into the equation of what we need to do about it. In setting policy, there are some things we have control over. But you need to balance your possible actions against what you really see."

The same goes for an individual's decisions. Whereas it is unethical to strap people down and expose them to air pollution in a lab, Cowin points out that subjects will essentially do it to themselves: "They'll stand behind a bus for 20 minutes chatting." With a smart sensor, he adds, we could decipher what triggered an asthma attack: "Were they standing behind a bus or Uncle Henry's smoking cigarette?" An answer may very well help defuse a serious health problem in the subject's future.