Portable pollution sensor displays air quality data on user's
18 December 2012
A portable pollution sensor that relays data to a smartphone
allows users to monitor air quality in real time. The device, called CitiSense, could
be particularly useful to people suffering from chronic conditions,
such as asthma, who need to avoid exposure to pollutants.
Computer scientists at the University of California, San Diego
have built a small fleet of the sensors. The aim
is to deploy sensors over a wide area to combine the data to
estimate air quality throughout the area where the devices are
deployed, and make the information to everyone, not just those carrying
The CitiSense data displayed on a smartphone
The CitiSense is a brick-sized box that contains three sensors to detect ozone, nitrogen dioxide and carbon
monoxide, the most common pollutants emitted by vehicles. The
user interface displays the sensor’s readings on a smart phone by
using a colour-coded scale for air quality based on the EPA’s air
quality ratings, from green (good) to purple (hazardous).
The CitiSense device
The developers say that just 100 of the sensors deployed in a fairly large area could
generate a wealth of data — well beyond what a small number of
EPA-mandated air-quality monitoring stations can provide. For
example, San Diego County has 3.1 million residents, 4,000 square
miles — and only about 10 stations.
“We want to get more data and better data, which we can provide
to the public,” said William Griswold, a computer science professor
at the Jacobs School of Engineering at UC San Diego and the lead
investigator on the project. “We are making the invisible visible.”
Researchers provided the sensors for four weeks to a total of 30
users, including commuters at UC San Diego and faculty, students and
staff members in the computer science department at the Jacobs
School of Engineering. Computer scientists presented findings from
these field tests at the Wireless Health 2012 conference in San
Diego earlier this year.
The sensors turned out to be great educational tools for their
users. Many people assume that pollution diffuses equally in the
air. But it actually remains concentrated in hot
spots, along main roads, at intersections and so on. The sensors
made this clear for users.
Wendy Chapman, an associate professor at
the UC San Diego School of Medicine, was one of them. She often bikes to work and discovered that pollution on her
route varies widely. She was exposed to the most pollution when she
used the bike path along State Route 56. But when she drove home on
that same road, she had virtually no exposure.
“The people who are doing the most to reduce emissions, by biking
or taking the bus, were the people who experienced the highest
levels of exposure to pollutants,” said Griswold.
Users discovered that pollution varied not only based on
location, but also on the time of the day. When Charles Elkan, a
professor in the Department of Computer Science and Engineering,
drove into work in mid-morning, the readings on his sensor were low.
But when he drove back home in rush hour in the afternoon, readings
were sometimes very high.
Elkan said being part of the study allowed him to gauge how
worried about pollution he should actually be. Air quality in San
Diego is fairly good, he added. “It’s a valuable study,” Elkan said. “I think it’s going to have
a big impact in the future.”
Elkan added that he could envision a day in the near future when
the sensors used by CitiSense would be built into smartphones,
allowing virtually everyone to keep tabs on the levels of pollution
they encounter every day. Of course, that means people might start
worrying more about pollution as something they can see and measure.
Many of the users in the study did take action to limit their
most severe exposure to pollutants. For example, bicyclists found
out that they could avoid a great deal of exposure by simply biking
one block away from a busy street. Commuters who took the bus
avoided waiting near the vehicle’s tail pipe, where the air quality
was poor. One user convinced his supervisor to install new air
filters in the office after registering poor air quality readings on
Researchers also noticed that the users were sharing the
information they collected, not only with family, friends and
colleagues but also with strangers who asked them about the sensors
during their commute or in public places. In other words, the
sensors turned cell phones into a conversation starter, rather than
devices that isolate their users from those around them.
The future of the project
Some of the sensors are currently on loan to researchers at San
Diego State University who are gauging air quality in San Ysidro, a
community right on the border between the United States and Mexico,
and one of the most polluted areas in San Diego County. Researchers
hope to secure a grant from the National Institutes of Health to
monitor air quality for school-age asthmatic children in that area
and to determine what can be done to limit their exposure to
The ultimate goal of CitiSense is to build and deploy a wireless
network in which hundreds of small environmental sensors carried by
the public rely on cell phones to shuttle information to central
computers where it will be analyzed, anonymized and delivered to
individuals, public health agencies and the community at large. The
sensors currently cost US$1,000 per unit, but could easily be
mass-produced at an affordable price. So far, Griswold’s team has
built and deployed 20 of them in the field.
Computer scientists used an artificial
intelligence method, called Latent Variable Gaussian Regression, to
capture high-quality data from the sensors in an uncontrolled
environment. The method allowed researchers to remove noise from the
data. “Sensors will differ. Sensors will fail,” Griswold explained.
“People will breathe on them. We wanted to make sure we got good
data in these conditions.”
Technical challenges remain. The data exchanges between smart
phones and sensors use up a great deal of the phones’ batteries.
During field tests, researchers provided users with two chargers — one
for home and one for work — to ensure that their phones were not going
to run out of power.
To extend battery life, researchers are experimenting with
uploading data from the sensors to the phones every 15 minutes or
only when the user wants to retrieve the information. Computer
scientists also have developed methods to turn off a phone’s GPS — a
huge drain of the devices’ batteries — when the device is immobile.
These innovations to extend battery life were made possible by
Krueger’s previous work in service-oriented architecture, which can
keep various components — like machine learning, power management and
security code — much more separate than in traditional software
systems, where functional elements are often so woven into the
source code that it is difficult to quickly update any one aspect of
CitiSense is funded by a $1.5 million grant from the National
Science Foundation. Qualcomm, Inc. donated funds for the cell phones
used for the project.