Until recently, measuring air pollution was a task that could be performed only by trained scientists using very sophisticated - and very expensive - equipment. That has changed with the rapid growth of small, inexpensive sensors that can be assembled by almost anyone. But an important question remains: Do these instruments measure what users think they are measuring?
A number of venture capital-backed startup or crowd-funded groups are marketing sensors by configuring a few dollars' worth of electronics and some intellectual property - mainly software - into aesthetically pleasing packages. The Air Quality Egg, the Tzoa and the Speck sensor are examples of gadgets that are growing in popularity for measuring air pollutants.
These devices make it possible for individuals without specialized training to monitor air quality. As an environmental health researcher, I'm happy to see that people are interested in clean air, especially because air pollution is closely linked with serious health effects. But there are important concerns about how well and how accurately these sensors work.
At their core, these devices rely on inexpensive, and often uncertain, measurement technologies. Someday small sensors costing less than US$100 may replace much more expensive research-grade instruments like those used by government regulators. But that day is likely to be far away.
New territory for a known technology
Pollution sensors that measure air contaminants have been on the market for many years. Passenger cars have sophisticated emission controls that rely on data collected by air sensors inside the vehicles. These inexpensive sensors use well-established chemical and physical methods - typically, electrochemistry or metal oxide resistance - to measure air contaminants in highly polluted conditions, such as inside the exhaust pipe of a passenger vehicle. And this information is used by the vehicle to improve performance.
It turns out that these sensors can work outside of your car too. But they have some important limits. They are often not designed to work in the open air, where conditions are much cleaner than in vehicle exhaust. And they can be affected by conditions such as varying temperatures or relative humidity, or the presence of interfering gases that they are not designed to measure.
Sensor manufacturers sometimes provide limited information on these low-cost sensors, and it is very easy to use the devices improperly. This is because they are designed to work under very controlled conditions - for example, at fixed temperatures or with limited wind movement - and these requirements often are not communicated to consumers. Measurement accuracy is especially important when we are trying to understand how exposure to air pollutants can lead to health problems. If we rely on poor measurements and reach incorrect conclusions, we will fail to protect public health.
In a recent commentary in Nature, British researchers Alastair Lewis and Peter Edwards highlighted many questions about using inexpensive sensors to measure air pollution. They conclude that these technologies must be better validated prior to general public use, and warn that academic investigators should not be gatekeepers for using them. Rather, what we can do is provide essential test beds to evaluate sensor performance through testing and calibration. We also can call on sensor manufacturers to explain these devices' limitations more clearly to customers.
Supporting citizen scientists
Many users of these sensor platforms are citizen scientists who have little formal training in measuring air quality. People are rightfully concerned about degraded air quality in their community, and are taking matters into their own hands by downloading open source plans, purchasing a few items and deploying their measurement systems.
They can do this with the help of inexpensive, open-source microprocessors and a growing library of open-source software. Agencies such as the U.S. Environmental Protection Agency and the South Coast Air Quality Management District in California, among others, recognize this growing interest. They also see the potential danger of empowering anyone to build and use tools that can produce highly inaccurate information.
Miniaturized versions of expensive sensors still can cost thousands of dollars - far out of reach for most citizen scientists. So it is likely that market forces and consumer convenience are driving the growth of a do-it-yourself sensor market. Whatever the motivation, these sensors are being used now by many organizations, including concerned citizen advocacy groups, and to some extent by the regulatory community. Regulators are interested in these technologies because they can cheaply expand measurement capacity. But at the same time, they are cautious because of lingering uncertainties about measurements that do not comply with narrowly prescribed measurement methods.
It is not hard to build a $30 sensor to measure carbon monoxide, although such a device probably will not be able to measure concentrations less than, say, one part per million. In many wealthy countries, where pollution levels are relatively low, such a device would not produce meaningful measurements. But on a busy street in New Delhi, or near a brick kiln in Nepal, it could be quite useful because pollution levels are significantly higher.
Low-cost air monitoring does have merits. For much of the world these tools could greatly increase understanding of pollution risks, especially in countries that do not have the financial resources or research infrastructure to produce sophisticated air quality measurements. Many environmental health scientists would like to expand the reach of these sensors to every corner of the world.
Use of DIY air sensors will continue to grow as people around the world learn more about the health risks of air pollution. The key is to make sure they work as reliably as possible. By expanding sound research measurements, we can continue to educate the public on the risks of air pollution, and to lobby for better protection from this hazard in a more informed way.