We operate a number of different monitoring sites looking at the following pollutants:

Nitrogen Dioxide

Nitrogen dioxide (NO2) is one of the most dangerous of the oxides of nitrogen to human health. It can be directly released into the environment or formed in secondary reactions in the atmosphere.

The main sources of nitrogen dioxide in the UK are road transport, energy generation and domestic and industrial combustion.In Bath and North East Somerset two methods to monitor Nitrogen Dioxide are used.

  • Diffusion tubes - which provide monthly average concentrations. Currently around 100 in Bath & North East Somerset.
  • Continuous analysers which measure the oxides of nitrogen using chemi-luminescence.

Particulate Matter (PM)

Particulate matter (PM) is classified according to size:

  • PM10 is particles that are less than or equal to 10 micrometres (µm) in diameter
  • PM2.5 is particles that are less than or equal to 2.5 micrometres (µm) in diameter

Particulate matter has a number of sources, both natural (sea spray and desert dust) and human made (smoke from fires and dust from tyres and brakes).

Primary PM is directly emitted from the source, whereas Secondary PM is formed in the atmosphere through chemical reactions between other pollutants.

BAM Tape


Ozone (O3) is a very reactive chemical formed by the reaction of nitrogen dioxide and sunlight.

Ozone is found at lowest concentrations next to busy roads where the release of nitric oxide (NO) from vehicles reacts with and removes the ozone.

The highest concentrations of ozone occur during the summer months when sunlight is at its strongest, and in rural areas where there is little nitric oxide from vehicles to remove the ozone.


Smoke is defined as a suspension of particles less than 5µm in diameter. Smoke can act as a carrier for other gaseous or liquid pollutants.

Monitoring of smoke and black carbon was carried out in Bath until 2011.

The city of Bath is a Smoke Control Area.  Smoke must not be discharged from domestic chimneys.  Further information is available on our Smoke Control webpage.

Sulphur Dioxide

Sulphur dioxide (SO2) is a corrosive, acidic gas that dissolves in water to form sulphuric acid; responsible form acid rain.

The main sources of sulphur dioxide is energy generation, industrial combustion and domestic burning.

Monitoring of Sulphur dioxide in B&NES ceased in December 2007 - all monitoring data was lower than the Government's Objectives.

Carbon Monoxide

Carbon monoxide (CO) is a very dangerous pollutant gas - it is colourless, tasteless and has no smell. Carbon Monoxide is produced in the process of incomplete combustion, when there is a deficiency of oxygen.

Carbon Monoxide is no longer monitored in Bath & North East Somerset, as the levels are significantly below the national objective limit. The reduction in levels is due to improving fuel and engine technology.


Benzene (C6H6) is a complex compound of carbon and hydrogen which is a liquid at room temperature but readily evaporates to emit small amounts into the atmosphere.

Benzene concentrations measured on London Road (Bath) have been lower than the Government objective of 5 micrograms per cubic metre (µg/m3) at all times.

Monitoring Sites

We operate four continuous monitoring sites providing up to date information on various pollutants.

  • London Road* - nitrogen dioxide and benzene

  • London Road Enclosure - particulate matter (PM10 and PM2.5) and nitrogen dioxide

  • Guildhall - nitrogen dioxide

  • Lower Bristol Road nr Windsor Bridge - nitrogen dioxide and particulate matter (PM10)

*For up to date monitoring information from the UK Air Quality Archive visit their website at  http://uk-air.defra.gov.uk.

There is also a network of diffusion tubes covering background and roadside locations.

We also operate two AQMesh analysers which are moved around the district - these provide indicative NO2 and PM measurements.

Monitoring enclosure at Lower Bristol Road nr Windsor Bridge NO2 diffusion tube on a lamppost in Odd Down, Bath

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