By Prof James Evans, Dr Thomas Bannan, Prof David Topping and Dr Jamie Anderson from Urban360

Reprinted from：https://airqualitynews.com

Every week seems to bring new research revealing the terrible health impacts of breathing polluted air. Poor air quality impacts children from before they are born up right up to adolescence, damaging lungs and brain function and making them more prone to chronic respiratory problems and allergies. Old people and those with health conditions are also more vulnerable. Indoor air quality, whether at home, when we travel in vehicles or in shared spaces like offices or schools can be poor too. This is especially bad, as we tend to spend more time indoors than outdoors but know very little about indoor air quality.

We are all affected. Recently the World Health Organisation reduced the recommended limits for different pollutants because evidence now indicates that there is no safe level of air pollution – any pollution in the air is harming us. But how can we find out how bad the air is that we are breathing? We need to understand the problem in order to be able to do something about it. Without monitoring air quality effectively, efforts to improve it can be misdirected, costly and ultimately ineffective.

This article shares three tips to help you monitor air quality effectively. With this knowledge you can monitor air quality in a reliable way in order to understand whether it is bad, and how to make effective changes to improve it.

First, know your pollutants. Air quality is a catchall for anything that is in the air and includes thousands of different things. Some of these things are gases. Nitrogen dioxide for example is produced by cars and is harmful to human lungs. PM stands for particulate matter and is detected based on its size. This is why people detect PM 1, PM 2.5 and PM 10. PM can be made up of bits of black carbon from burning in combustion engines or stoves, or biological matter like pollen or fungal spores.

Each kind of pollutant is different so should be monitored differently. PM 2.5 tends to distribute evenly across entire regions, whereas nitrogen dioxide differs depending whether you are next to a busy road or a few metres from it. If you are monitoring a scheme to reduce traffic, then you need sensors that can detect nitrogen dioxide next to affected roads. Our work looking at the impact of low traffic neighbourhoods used as many sensors as possible to detect changes in levels of nitrogen dioxide from traffic on roads that were of concern to residents. Being able to detect fungal spores is more important when monitoring indoor air quality, as they indicate a damp problem that can damage health. Air purifiers with uncleaned filters can even harbour fungal growth and inadvertently spread spores into the air.

Second, know your sensors. This is critical in order to be able to design monitoring schemes that will answer the question you want answered for the budget you have. A number of projects asked us to measure the effectiveness of air purifiers in schools and care settings. We used sensors to monitor differences between the indoor and outdoor air quality, and between classrooms both with and without purifiers. For this question, it was more important to have a number of reliable sensors in different locations measuring particulate matter that air purifiers filter. Indoor sensors are often cheaper as they do not need to be weather proofed, but require buy in from building users to make sure they aren’t turned off or moved. Spoiler alert, our results show that purifiers do reduce particulate matter by about 20% during school hours, and are more effective out of school hours when people are not coming and going. 

Other monitoring schemes are designed to understand the impacts of specific interventions, like pedestrianisation or traffic calming schemes. In these cases, it is important to install sensors before the change takes place to compare before and after. In these schemes, we also use traffic sensors located at the same places as the air quality sensors to add some certainty that any observed changes in air quality are due to changes in traffic.

Finally, know your data. Is it reliable? Air quality sensors are fragile and even expensive equipment needs testing regularly to check it is producing accurate readings. This is calibration, and involves placing sensors for long periods at scientific air quality monitoring stations to check they are giving the same readings. Without calibration sensors can ‘drift’, whereby the data they provide is wrong but not so wrong as to not be believable. The UK government recently launched MCERTS, which is a national certification for air quality sensors that offers a guarantee of their reliability. Even with this, if air quality data is to be used in science or as legally acceptable evidence for making decisions the people responsible for collecting it must be able to show they have robust Quality Assurance and Quality Control processes in place.

Once you are confident that the data you are collecting is accurate the next step is to understand how to analyse it. Raw air quality data means very little by itself. For example, during periods of high pressure when there is little wind, air pollution tends to build up. When the weather is windier it tends to disperse. Air quality data can be adjusted to reflect this through a process called weather normalisation, or can be collected over long enough periods to provide reasonable comparisons. For example, for highways projects that are changing traffic flows we like to collect 12 months of baseline data and at least 12 months of data after the change has been implemented. Comparing between 12 month periods removes some of the impact of the different seasons on the results.

Sometimes data is already available, and this can reduce the costs of monitoring air quality. DEFRA operate 174 air quality sites that give reliable data across the UK at relatively large scales. Some transport authorities make traffic data openly available, although only at sites they have existing cameras. Comparing sites you are monitoring with other similar sites for which data already exists can be a great way to understand the relative air quality. Weather data is available, often at a cost. We have helped local authorities identify existing sensors and data that can massively reduce the cost of monitoring.

Monitoring air quality is becoming increasingly important to evaluate the effectiveness of different kinds of schemes and technologies to tackle air pollution. Indoor air quality is becoming a critically important challenge for homeowners, building operators and service providers. From highways schemes to green screens around playgrounds, and from air purifiers to building ventilation systems, we need more monitoring of air quality in order to diagnose problems accurately and fix them quickly.