Scientists know, based on data from eight monitoring stations in and around Baltimore, that smog levels are not healthy on many hot summer days.
But the air on some blocks is surely fresher than others.
Hundreds of simpler air pollution sensors will soon be scattered around the city to show where those oases lie, and what about them makes the air healthier to breathe.
The solar-powered plastic cubes, each about half the size of a toaster, will provide new data every day on levels of ozone and nitrogen dioxide, pollutants known to attack and weaken lung tissue.
The effort, dubbed Baltimore Open Air, is an example of what environmentalists and regulators hope will be a wave of "citizen science" projects using relatively cheap and portable technology to detail the reaches of air pollution.
Existing technology is prized for its precision and accuracy, but the fleet of new sensors designed by Johns Hopkins engineers — and built with the help of four city residents through the nonprofit Civic Works program — will paint a clearer picture of how air quality varies from one neighborhood to the next.
"It's a new way of doing things," said Anna Scott, a doctoral student in earth and planetary science at the Johns Hopkins University who spearheaded the project. "This is kind of a big experiment."
Though air quality has improved along the Baltimore-Washington corridor over recent years, the region still falls behind some ever-tightening federal and state pollution limits, and Baltimore City's asthma rates are some of the highest in the country.
George S. "Tad" Aburn, director of the Maryland Department of the Environment division that oversees air pollution, said he is eager to see data from the Open Air network, and similar projects that could follow it. Environment Secretary Ben Grumbles is helping to lead a national committee exploring similar uses of relatively simple technology.
"I think it is, over the next five years, going to change some of the things we've historically done," Aburn said. "I think we're going to learn a lot."
The fleet of about 250 air monitors is tediously coming together in a hot warehouse in the city's Remington neighborhood. Scott and colleagues, including four assemblers hired through the Civic Works job-training program, are in the midst of a three-week manufacturing process that could have the sensors deployed around Baltimore this fall.
Each sensor is contained within a cube of white plastic. A tiny fan sucks air inside, where it meets temperature and humidity gauges and a circuit board with four sensors that collect readings on pollution levels, explained Chris Kelley, a doctoral student in environmental engineering working with Scott, who designed the guts of the monitors.
A small solar panel on the outside recharges a battery that holds twice as much energy as a typical smartphone power source, enough to keep the contraption running through five cloudy days.
On a recent morning, Thurman "Dominic" Brown hot-glued strips of foam onto openings that will serve as the sensors' vents, then passed them along to colleague Charles Epperson, who used a soldering iron to attach the opening into which air will flow.
Both men, in their early 30s, previously trained in Civic Works programs that teach city residents to work in home weatherization, solar panel installation or stormwater management.
"I'm a quick learner," said Epperson, whose sister gave him a Civic Works pamphlet when he was looking for work. For now, he's making $15 an hour building the air sensors. Once that project is done later this month, he expects he'll return to home weatherization, perhaps starting his own insulation business.
The sensors are bound for sites across the city and around it. Neighborhood associations and groups such as the Chesapeake Bay Foundation have volunteered to house them, as has a woman in Anne Arundel County who said she wants to put one on her dock.
Scott hopes to have coverage throughout the area, and plans to fill any gaps in the map by installing cubes at city library branches.
The network is not as sophisticated as existing pollution-monitoring technology designed to meet strict rules laid out in federal regulations. The MDE has a network of two dozen such stations across the state, capable of detecting a single particle of any given pollutant within a billion particles of air.
That type of infrastructure costs anywhere from tens of thousands of dollars to hundreds of thousands. The state's most intricate station, in Beltsville, cost about half a million dollars, Aburn said.
The Baltimore Open Air project, on the other hand, has a $70,000 budget.
The team landed $40,000 through an Environmental Protection Agency competition called the Smart City Air Challenge — the proposal was one of two chosen for funding, the other in Lafayette, La. — and got matching funds from the Environment, Energy, Sustainability and Health Institute at Hopkins.
They also crowdfunded $2,600 through the website Experiment.com at an early stage in their research, as they looked at how parks and trees provide cooling within Baltimore's urban heat island.
The lower price tag means the instruments won't be as sensitive as the technology used under federal regulations. But for the researchers' purposes, that's OK.
"We're more interested in relative air quality," said Yan Azdoud, who recently finished a postdoctoral fellowship in mechanical engineering at Hopkins and is working alongside Scott and Kelley. They hope to end up with a high-resolution map showing how air quality varies around the city.
Another group of Hopkins researchers is working on a similar project, a partnership with Yale University. The SEARCH Center, exploring "Solutions for Energy, Air, Climate and Health," plans to install 40 or 50 monitors around the city this fall to investigate ways changes in energy technology are affecting air pollution.
The effort is using similar technology as Scott's team, but will track more types of pollutants and put a bigger emphasis on precision and accuracy, said Kirsten Koehler, an associate professor of environmental health and engineering at Hopkins' Bloomberg School of Public Health.
Projects using simple technology — some of which can even be bought on Amazon.com — don't replace sophisticated, expensive monitoring equipment, but make it possible to gather data on a broader scale than was previously possible, she said.
"There's definitely a boom of this kind of lower-cost sensor technology," Koehler said. "We can start to understand the variation between cities, but also within a city."
The benefit of that sort of information, Scott said, is that it can help policymakers address air quality on a hyper-local level.
“That’s a huge improvement over anything else,” she said.