Breathing space: the facts behind cycling and air pollution in Cambridge

This article was published in 2020, in Magazine 146.

Pollution trap: Great Northern Road is an example of a ‘street canyon’.
Image as described adjacent

Air pollution is an issue that is receiving increased media coverage and is becoming a greater part of the public consciousness. But how bad is it in Cambridge, what should we be doing about it, and how does cycling fit in?

What is air pollution?

The main pollutants of concern in urban areas in the UK are nitrogen dioxide (NO2) and particulate matter (PM10 and PM2.5). PM10 refers to particulates with a diameter of less than 10μm and PM2.5 refers to particles with a diameter of less than 2.5μm. Particulate matter is made up of a mixture of substances including elemental and organic carbon, metals, mineral compounds, sulphates, nitrates and salt.

Where does air pollution come from?

The major sources of NO2 are combustion processes including road traffic, domestic heating, industrial processes and power generation.

PM10 and PM2.5 are generated by combustion processes, but also come from a wide range of other sources including vehicle brake- and tyre-wear, road-wear, construction activities, natural sources and the formation of sulphate and nitrate particles by chemical processes in the atmosphere.

Levels of pollution in the air we breathe depend not only on the quantity of the pollutants that are released, but also on the dispersion of the pollutants between the source and us. This is influenced by the nature of the release, the meteorological conditions and the physical environment.

Emissions from road traffic are particularly significant because they are released close to the point of exposure and street canyons – where roads are lined by buildings on both sides – can trap pollutants and significantly worsen air quality.

What are the health impacts?

Public Health England states that poor air quality is the largest environmental risk to public health in the UK.1 NO2 has long and short-term health impacts. High levels can inflame the airways in the lungs and affect how well the lungs work. People with asthma are particularly affected.

Exposure to fine particulate matter (PM2.5) can lead to increased admissions to hospital and mortality of people suffering from heart disease (attacks and strokes) and lung disease such as Chronic Obstructive Pulmonary Disease (COPD), bronchitis and asthma.2 It is recognised that there are no absolutely safe levels of particulate matter. Evidence suggests that health effects can still occur within the legal limits and that any improvement in air quality will have positive health consequences.3

Children, the elderly and people with respiratory and cardiovascular disease are known to be more susceptible to the health impacts of air quality. Children are especially vulnerable because they breathe more air relative to their body weight than adults, are more physically active and tend to be outdoors at times of higher air pollution levels.

Air quality limits and objectives

The EU has set legally binding limit values for concentrations of NO2 and PM10 and a target value for concentrations of PM2.5. These are incorporated into the UK National Air Quality Objectives. The World Health Organisation (WHO) sets global guideline values for pollutant concentrations. The guideline values for PM10 and PM2.5 are more ambitious than the UK National Air Quality Objectives.

How bad is air pollution in Cambridge?

The good news is that air quality throughout the UK has improved considerably over recent decades, but concentrations of NO2 remain above the air quality objectives in many urban areas. The environmental law charity Client Earth has successfully taken the UK government to court three times over its failure to improve air quality quickly enough.

Air quality varies considerably throughout Cambridge. In suburban areas and away from busy roads, concentrations of NO2 are well below the National Air Quality Objectives, but annual average concentrations are likely to exceed the National Air Quality Objective along some of the busiest roads in the city.

Particulate matter levels are below the National Air Quality Objectives throughout the city, but in parts of the city they are above the WHO guideline values.

Measured levels of NO2 in Cambridge in 2018 were, overall, slightly lower than in 2017. Measured levels of particulate matter in 2018 had risen slightly (PM10) or remained the same (PM2.5).4

In Cambridge and South Cambridgeshire, 106 deaths per year can be attributed to poor air quality.5

What is being done to improve air quality?

Air quality in Cambridge is the responsibility of the city council. In 2004, it declared an Air Quality Management Area (AQMA) covering the centre of the city, owing to levels of NO2 exceeding the National Air Quality Objective. The council measures NO2, PM10 and PM2.5 at a set of fixed monitoring sites to test compliance with the air quality objectives and to provide information about how air quality is changing over time.

The city council developed a new Air Quality Action Plan (AQAP) in 2017, which sets out the council’s priorities for improving areas of poor air quality, and maintaining areas of good air quality. It includes a list of actions with timescales and the expected improvement in air quality that the action would deliver.

The proposed actions fall into three main categories:

  1. Reducing local traffic emissions as quickly as possible to meet national air quality objectives,
  2. Maintaining levels of pollutants below national air quality objectives,
  3. Improving public health.

How can cycling help?

Cycling can play an important role in all three of the categories set out in the Air Quality Action Plan.

1) The most effective way of reducing local traffic emissions is to remove vehicles from the road. The AQAP includes a number of measures which aim to encourage modal shift for both personal travel and business journeys. These include new and improved cycle routes, potential incentives for cycle deliveries and travel planning. A workplace parking levy and roaduser charging are also currently being discussed .

2) Where pollutants are already below national air quality objectives, the AQAP aims to maintain these levels by using planning policies to ensure new communities are designed to make it easy for people to use sustainable modes of transport.

3) Swapping car journeys for cycling can improve public health for all city residents by reducing levels of pollution. It also has health benefits for the cyclist through increased physical activity.

What about electric cars?

Swapping conventional vehicles for electric vehicles can help to significantly reduce concentrations of NO2. The impact on particulate concentrations is likely to be much smaller, as electric vehicles will continue to generate significant emissions of particulates from brake, tyre and road wear. Switching to electric vehicles will also not have any of the other benefits of cycling such as reducing congestion, increasing physical activity and improving safety.

Should I be worried about air quality when cycling?

A study carried out by the Centre for Diet and Activity Research (CEDAR) and Medical Research Council Epidemiology Unit at the University of Cambridge found that, for most people, the health benefits of walking and cycling outweigh the negative health effects of air pollution, even in cities with high levels of air pollution. In fact, car drivers can be exposed to more air pollution than cyclists6. Exposure to air pollution can be reduced by planning your route to avoid busy roads, where possible.

Matt Williams
Member of the Institute of Air Quality Management


  1. Public Health England, 2018.
  2. Environmental Protection UK.
  3. Air Quality: A briefing for directors of public health, Local Government Association, 2017.
  4. Cambridge City Council Air Quality Status Report 2019.
  5. Cambridge City Council Clean Air Zone Feasibility Study
  6. Levels of ambient air pollution according to mode of transport: a systematic review, The Lancet, 2016.