Chippenham Air Quality fails World Health Organisation Recommended limits

We’ve been a little quiet on our regular Chippenham Air Quality updates – but now we have a big one! An analysis from October 2025 to August 2026. The recent newspaper reports on some of the medical research outcomes of ULEZ have shown just how important air quality is for human health, and particularly children.

Background

In December 2025 Zero Chippenham took a stall at the Emery Gate Christmas market. We met some wonderful people that day including a local Data Scientist – Dr Jack Hunt. Jack was interested in our Air Quality project and offered to help analyse the data. As we had just switched to using the Wiltshire Council Purple Air monitors it seemed the perfect opportunity to analyse the larger sensor network now in place in Chippenham. We’ve had regular updates from Jack at Zero Chippenham meetings and he’s been working on multiple methods of modelling the data and even predicting future air quality. So here are the results on just how clean Chippenham’s air has been from October 2025 to August 2026.

What are we measuring and what are the limits?

We are measuring Particulate pollution, non gaseous particles in the atmosphere. PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometres or smaller, small enough to penetrate deep into the lungs and even enter the bloodstream, posing significant health risks.

What are the limits for particulate pollution?

The Air Quality Standards Regulations 2010 require that concentrations of PM in the UK must not exceed:

An annual average of 40 µg/m³ for PM10;
A 24-hour average of 50 µg/m³ more than 35 times in a single year for PM10;
An annual average of 20 µg/m³ for PM2.5.

The World Health Organisation(WHO) has set tighter limits for particulates. The current guidelines state that annual average concentrations of PM_2.5 should not exceed 5 µg/m³, while 24-hour average exposures should not exceed 15 µg/m³ more than 3 – 4 days per year.

Chippenham Air Quality October 2025 to August 2026

The plots below are the cumulative days in which we exceed the WHO 24hr PM2.5 recommended exposure limit, both for all sensors and averaged across sensors.

The following plots show PM2.5 concentration with the WHO recommended limit indicated by the dashed horizontal line and days in which the limit is exceeded indicated by the vertical red lines. Note that the shading represents two standard deviations around the mean and that PM2.5 concentration cannot be negative; the shading is to indicate the variability in the 24hr estimates.

Predicting Air Quality

Jack has been experimenting with modelling techniques to see if it is possible to predict or forecast air quality in Chippenham using historic data and meteorological data such as air pressure and humidity. Jack has had some success predicting over a short time period of one or two days, but less accuracy over a longer period.

Jack says…

“The ar_mean plot (and its associated R^2 distributions – distributions because this is Bayesian stats) shows the use of the previous 8 hours observations to predict the following 4. Note that the configuration here is tweakable, we can forecast longer using more observations back in time. That isn’t actually the interesting part. What is interesting are the ar_agg plots, which attempt to use the last 5 days of 24hr averages to predict (in the same autoregressive process approach) the next 2 – this model fails to fit (see the poor R^2 distributions and the generally poor predictions). This indicates that the temporal correlation (at least that can be modelled linearly by lagged observations) is stronger over shorter time periods – again, nothing groundbreaking here and perhaps isn’t that surprising if you think about driving patterns etc through an average day. This holds with the earlier plots I sent you indicating a short term periodicity.”

The red line is predicted and the blue line is the measurement.

Conclusions

Based on the analysis period Chippenham meets the UK limits for PM2.5 particulate pollution, but far exceeds the World Health Organisation (WHO) guideline to not exceed 15 µg/m³ more than 3 – 4 days per year. In Chippenham we have already exceeded this guideline for 21 days averaged across all Chippenham sensors. This doesn’t include the recent period of Welsh wildfires where pollution was extremely high.

It’s also interesting to note that the general patterns (aside from one off, localised events) are consistent across the sensors in the Wiltshire Community Air Network (WCAN) programme, so this implies that the air quality is broadly consistent across the project area.

It may be possible to predict or forecast air quality for a short period and potentially use this to warn of poor air quality or provide an automated forecast with some caveats.

Why does this Matter?

Earlier this week it was reported in the Guardian Newspaper [1][2] how the introduction of ULEZ in London led to an the increase in lung capacity in children.

Children’s lungs grew bigger and stronger after the most polluting vehicles were restricted from entering London, a study has found.

Examinations of London schoolchildren before and after the introduction of the city’s ultra-low emission zone (Ulez) found a restoration of lung capacity that had been stunted by exposure to pollution.

The researchers said the findings were a vindication of the low-emission zone policy, which had been criticised as a burden on motorists.

“Traffic pollution in cities damages children’s health and development,” said Chris Griffiths, a professor of primary care at the University of Oxford and Queen Mary University of London (QMUL), and the study’s joint senior author. [2]

The air we breathe has a direct effect on our health and it is important to understand the causes. It has also been shown that improving air quality improves human health, especially the health of our children.

[1] Ulez led to better lung size and function among London children, study finds

[2] Impact of the Ultra Low Emission Zone on lung function growth in children in London, UK: a prospective parallel cohort study

Do you have a Wiltshire Council Purple Air monitor?

Wiltshire council are bringing their Purple Air monitor project to a close shortly. Though the sensors will remain active, it is Zero Chippenham’s understanding that support for the sensors is likely to end. If you have a Purple Air sensor in the Chippenham area Zero Chippenham would very much like to support you to continue operating the sensor. In the first instance please contact the Wiltshire Council Air Quality team that supplied your with the sensor. However if you no longer wish to keep the sensor at your house, or need support to keep it running, our volunteer network would be pleased to work with you and Wiltshire Council to either help you keep the sensor running, or re-home the sensor to one of our Chippenham volunteers.

What are the possible causes of Air Pollution in Chippenham?

The following considers what the potential causes of particulate air pollution in Chippenham might be, and what we can do about it – if anything. It’s a conversation starter rather than a definitive statement…

The peaks of pollution we see will be one off, perhaps unpredictable events, such as the Saharan dust earlier in the year – there are a bunch of the red lines around that time; Wildfires – although we don’t have any in these plots as they finish before the recent fires; and perhaps events in the town drawing more traffic. Though we would want to cross reference with the town through the year; it might not be significant. In our earlier work with just four sensors and before Jack joined the project, we could perhaps correlate roadworks in an area of town with a temporary increase in pollution. However we didn’t have the bigger sensor picture to definitively correlate this.

What will be more interesting will be seeing how the mean levels behave over a few years; is the situation getting better or worse? Ongoing monitoring will likely lead to more interesting insights than one off events will as they are often beyond our control.

In terms of what’s driving the mean (not obviously some event like a dust storm), that is very difficult to say. We could hazard a guess that typical town traffic would contribute. Localised log burning burning could be an issue, however we would probably want to ascertain which sensors are close to a home with a log burner and which are not, then we could drill into that a bit more. Of course, smoke and particulate matter will diffuse, so there doesn’t necessarily have to be a source very local to a sensor for it to have an effect; it will just be far harder to identify than if it were very local and presenting as a massive spike.

In terms of traffic the move to electric vehicles will remove tail pipe emissions, though particulates from tyres are still a potential particulate source.

We also have ongoing construction work in the Chippenham area as the town expands including the likely future demolition and rebuild of Emery Gate in the town centre. There is a gas fired Peaker power station on Cockleberry Lane and we have some industrial manufacturing nearby.

Jack has linked with weather data for the time period and has attempted to fit many models using variables such as wind speed and direction to explain PM2.5 concentrations in the town. None of these showed any indication that levels in the town are influenced by these variables, suggesting that we are not seeing significant pollution blowing in from elsewhere. However, we should note that in the literature, this has been the case for very polluted cities (in developing countries), where there is significantly more pollution to go around! Although we do not see it in the analysis, that does not mean there is no effect – just that we have have failed to observe it in the analysis, so there may simply not be enough signal to establish that correlation.

In terms of what we can do about it, that is perhaps a communication issue; people need to make choices for their health, but to do that they need to understand that there is a danger in the first place. As the ULEZ study is fresh, perhaps a first step is to read the lancet paper and ascertain how much worse than here London is, and secondly come up with some key facts about the health impacts (and improvements since ULEZ) of pollution. This could all feed into a local information campaign. Of course, there will be the argument that London is obviously worse than Chippenham, but we know that our exposure is above WHO guidelines so there is room for improvement and it should be clear that any exposure is bad, the ULEZ study being a more extreme example.