London smog
A cyclist on London's the Mall wears a pollution mask. Air pollution claims the lives of about 40,000 people a year in the UK. Reuters

Today is the first ever National Clean Air Day. Thousands of people across the UK are taking action to push for cleaner air, particularly in our cities.

Poor air quality has become an increasing concern in urban areas. The Royal College of Physicians states that every year, about 29,000 deaths die in the UK as a result of inhaling particulates. This figure may rise to around 40,000 deaths when also considering nitrogen dioxide pollution.

Public worries about these health effects have led people to take action to reduce air pollution – hence Clean Air Day – and also to monitor air quality for themselves, so they can see how much pollution they are being exposed to. But many of personal sensors for air pollution aren't up to the job.

Advanced, government-run monitoring stations provide valuable data on the air we breathe on busy streets, but it's impossible to have one on every corner. Unlike these monitoring stations, businesses are offering consumers the ability to track their exposure to air pollution wherever they go with portable air quality sensors.

Increasingly, local groups and individuals are buying them to measure their own or their children's exposure to pollution. They're used to lobby for traffic reduction measures in roads that pass by school playgrounds, for example. Low-cost sensors are also heralded as a way for governments to add to their existing network of data points and assess what difference their policies are having on the environment.

But a number of recent reports have questioned the quality of the data they provide and urged caution about how these sensors are used. Different sensors often give different readings in the same location, even if they're made by the same provider.

pigeon air patrol london pollution
London pigeons have been fitted with special monitors to help measure the capital's pollution levels. Monitors that people buy commercially are often very unreliable. DigitasLBi

Another problem is that sensor data quality can deteriorate over time. Traditional air quality monitoring stations that contain large, expensive instrumentation have regular checks to make sure they're working properly. Many portable sensors people buy are not subject to any ongoing checks of their performance or accuracy once they are in use. So there's no way for users to know if they're still working or not.

If people choose to buy these sensors, they should be aware of these limitations. Current scientific research suggests that these sensors are useful to determine whether air pollution goes up or down in one location over a period of up to a few months, or which of two locations is more polluted in the short-term. But they cannot provide a long-term indication of changes.

There is currently no compelling scientific evidence that these sensors are able to give absolute measures of concentration that could be used, for instance, to predict health effects. Equally there is little evidence that they could be used with confidence over prolonged periods of time.

Of course, they're not the only piece of equipment that consumers use that are not regularly checked or re-calibrated. You may ask why should I believe my bathroom scales but exercise caution with air quality sensors?

Firstly, bathroom scales have been around for a long time. The technology is well known and understood. Air quality sensors are new.

Secondly, bathroom scales do not suffer from interferences. It doesn't matter how hot the room is or how humid it is, they work the same. Air quality sensors are affected by the overall conditions and can give different readings of the same pollution levels if, for example, the weather is different, as they can be affected by humidity.

London smog
London typically breaches its annual pollution limit in the first week of the year. Reuters

Thirdly, we can judge whether our bathroom scales are starting to give the wrong result, as we have a reasonable sense of whether we are heavier or lighter than before. Doing this kind of sanity check is incredibly tough with air quality sensors.

For a start, the quantity of pollution the sensors are measuring is very small. Even in urban centres, the sensors are measuring around one unit of pollution to 100,000 units of clean air. Relative to our weight, this is like trying to detect a difference of 1g on bathroom scales. Add to that the fact that you cannot usually see the pollution, because most of it is invisible, and the job of sanity checking is made near impossible.

Solving these problems is the current focus of scientists and sensor manufacturers. Low cost air quality sensors may have the potential to help us measure personal exposure, increase the data we have about air quality in many different locations, and help us better understand the links between pollution and health impacts.

However, until the crucial issues with them are resolved, their use should be restricted to applications where research has demonstrated that they are fit-for-purpose. For now, people should not take the data they provide at face value for important decisions about their own lives.

So, on this Clean Air Day, do try to understand and minimise your exposure, but also take any data from low cost sensors with a pinch of salt.

Jane Burston is head of energy and environment, and Richard Brown is knowledge leader for the environment division, at the UK's National Physical Laboratory.