In 2010, a minor traffic lane change shifted Copenhagen’s air quality, underscoring how small spatial adjustments can significantly affect pollution exposure. Research using low-cost sensors reveals that personal choices, such as walking routes and what floor an apartment is on, can dramatically alter people’s exposure to harmful pollutants such as fine particles, nitrogen dioxide and ozone. Experiments demonstrated that minor routine changes could improve health outcomes, highlighting the importance of assessing hyperlocal air pollution levels in urban environments.
In 2010, an air quality monitoring station in downtown Copenhagen reported something alarming: the concentration of NO2, an air pollutant that irritates airways, leapt about 20% overnight. Had a new source of pollution emerged in that area?
Environmental scientists soon identified the culprit. “It turned out that one of the traffic lanes had been shifted in the street” next to the monitoring station, explains Ole Hertel, an environmental engineer at Aarhus University. “They made a turn lane, which meant the traffic went closer to the monitoring station.”
Air quality in Copenhagen had not radically changed, but this episode underscores how minute changes, on the scale of a metre or two, can dramatically affect local air quality. That also plays out in the small differences that distinguish you from your neighbours – such as what floor your apartment is on or which side of the street you choose for your afternoon stroll.
Researchers at Aarhus University are using low-cost sensors to assess how these small differences can affect your personal exposure to air pollution.
The initial results from this research reveal that “people can really reduce their personal exposure” by changing their routines slightly, says Louise Frederickson, an environmental science postdoc at Aarhus University and lead author of two new articles on assessing hyperlocal air pollution.
Low-cost sensors and airway irritants
In both studies, Frederickson and her team measured the concentrations of three air pollutants – fine particles, NO2 and ozone (O3).
Fine particles are the most insidious of the three, according to co-author Hertel. “Fine particles cause 6% of deaths in Europe,” Hertel says. “If they are small enough,” generally below 2.5 microns, the tiny particles of dust, smoke and liquid droplets “can get into your lungs and cause damage. A small fraction of mainly ultrafine particles can even pass into your bloodstream. In our body they can cause oxidative stress, inflammation and lead to type II diabetes, various types of cardiovascular disease, effect birthweight of babies, affect fertility and on long term also lead to lung cancer and other types of cancer,” Frederickson adds.
NO2 and O3 are “airway irritants” that can worsen symptoms of asthma and wear down the lung’s immune defences, Hertel says. “We usually consider NO2 together with ozone (O3) the second-most important air pollutant” for health.
Fixed monitoring stations mounted on top of buildings are used for the vast majority of air quality monitoring, generating the data for estimating how much pollution people are exposed to.
Although these bulky apparatuses are very accurate and important for detecting city-level changes to air quality, researchers are increasingly aware that they do not reflect the conditions individuals experience – after all, the average person does not spend that much time on rooftops.
Frederickson advocates measuring hyperlocal pollution using low-cost sensors. What these smaller sensors lack in accuracy compared with their roof-mounted cousins, they make up for in versatility. “They have very, very high time resolution,” Frederickson says. “And because they are portable, you can walk around with them and detect these differences at a very fine scale in an urban environment.”
Choices matter – except when they do not
In a new suite of experiments, Frederickson and her team put the low-cost sensors through their paces in Copenhagen daily life.
In the first experiment, Frederickson and her team tested air quality on two routes around the Copenhagen lakes. On a GPS receiver, the routes look identical – the only difference is that one experimenter walked on a sidewalk at street level, separated from traffic by a cycle track. The second route was slightly lower on a gravel path right by the lake.
The experimenters wore backpacks rigged with nine of the low-cost sensors – three for each of the major pollutants – and walked the paths during the morning and afternoon rush hours for five weeks.
The difference between the two paths, separated by just a metre or two, was striking. “Just taking the route close to the lake would halve your NO2 exposure,” Frederickson says.
But “we also saw that the pollution levels between the days vary more than the difference between the routes,” Frederickson adds. “This just highlights how much we cannot control in our personal exposure.”
However, reducing your cumulative personal exposure is a game of averages, she emphasises. If you walk the Copenhagen lakes every day anyway, a simple shift in habit could improve your health.
A second experiment featured air quality sensors mounted on five floors of a building on H.C. Andersens Boulevard, a busy thoroughfare in downtown Copenhagen. Using cameras and artificial intelligence vehicle recognition to monitor the flow of traffic, Frederickson and her team aimed to assess how much air pollution from passing cars reached the upper floors versus the lower floors.
Frederickson’s analysis of data collected over 100 days determined that the most important factor for this building was not the upper versus lower floors but rather the weather – a combination of humidity, temperature and wind velocity – followed by the day of the week and the time of day.
She could not identify any vertical gradient, although she emphasises that one reason may be because the building is a generous 18 metres from car traffic, separated by an expanse of grass and cycle tracks and pedestrian lanes. “It could be that all the road pollution is basically at the background city level when it gets to the sensor system,” or that the gradient is too subtle for the low-cost sensors to detect, Frederickson says.
With the H.C. Andersens Boulevard experiment as proof of concept for the set-up, Hertel says that future experiments with the low-cost sensors should investigate a “narrower street canyon” to help to elucidate the how floor height affects hyperlocal exposure to air pollution. “For instance, Jagtvej, near central Copenhagen, is very narrow and the buildings are taller than the width of the street.”
Should I open the window?
While Frederickson and her team continue to parse out the data with low-cost sensors, what should city dwellers do? Hertel says that they already have one take-home message for the public: “When you ventilate your home or office, always open the windows to the backside of the building and not to the street side generally,” he explains. “You will get much less polluted air into the building.”
