The Environmental Protection Team operates a number of individual diffusion tube sites throughout the borough. These diffusion tubes measure the amount of nitrogen dioxide in the air we breathe.

The quality of the air we breathe is important for our health and wellbeing. During large scale events such as the fire at the fuel depot site at Buncefield, the pollution of the air is a very obvious and visible occurrence. However, pollution from traffic and industry is often not visible.

We are a member of the Hertfordshire and Bedfordshire Air Pollution Monitoring Network (HBAPMN). 

All local authorities have been assessing air quality across their areas and comparing it against national objectives, set out in the Government's Clean Air Strategy, for a number of pollutants including carbon monoxide, benzene, 1,3 butadiene, lead, nitrogen dioxide, particulate matter and sulphur dioxide. Where air quality is poor an Air Quality Management Area (AQMA) is declared and, where possible, local action taken to improve the air in that area. Hertsmere currently have six declared AQMAs due to the levels of nitrogen dioxide.

In 2019 the Government produced The Clean Air Strategy 2019, which sets out the comprehensive actions required across all parts of government and society to improve air quality.

The strategy sets out how we will:

  • protect the nation’s health
  • protect the environment
  • secure clean growth and innovation
  • reduce emissions from transport, homes, farming and industry
  • monitor our progress


It complements three other UK government strategies: the Industrial Strategy, the Clean Growth Strategy and the 25 Year Environment Plan

Outlined in the strategy are the air quality objectives for a number of pollutants the Government feels are of most concern at present. Some of these are detailed below. 

PM10 Particulates

PM10 describes the fraction of airborne particulate matter that is less than 10 microns in size. These fine particles are of the greatest concern since they are capable of being easily transported over long distances on currents of air. Also, fine particles may be drawn into the lungs where they may adversely affect health. Recently, the attention of scientists has been drawn toward studying the PM2.5 fraction and even smaller particles, which can penetrate the very deepest parts of the lung.

PM10 and other particulate matter may vary considerably in chemical and physical composition. The principal sources of these particles are combustion processes, including traffic and industry.

Nitrogen Dioxide (NO2)

Nitrogen dioxide is one of a number of nitrogen oxides, which are formed during high temperature combustion processes. Road traffic is the main source, accounting for approximately 50% of all European emissions. Therefore, concentrations tend to be highest in urban environments with high traffic levels. Large industrial sources can also have a significant impact. In Hertsmere, almost all N02 comes from transport based emissions.

Nitrogen dioxide is a respiratory irritant and also plays a part in the production of another atmospheric pollutant, Ozone. Nitrogen oxides remain in the atmosphere for approximately one day before they are oxidised to nitric acid. Nitrogen oxides are therefore a contributory factor in the production of acid rain. 


Ozone (O3)

Ozone is a very reactive chemical, which is potentially toxic to both plants and animals. Up in the stratosphere, ozone helps to protect the earth from the harmful effects of ultra-violet rays from the sun. However at ground level it is a pollutant. Unlike the other pollutants mentioned above, ozone is not emitted directly. Rather, it is formed as a result of a complex series of reactions involving hydrocarbons, sunlight and nitrogen dioxide (NO2). The involvement of sunlight in this process means that ozone levels tend to be highest in summer.

The reactions take time to generate ozone and the highest concentrations are frequently experienced many miles away from the source of the pollution, perhaps in rural areas. In fact, a significant proportion of ozone incidents experienced in the UK are due to pollution imported from abroad. The problem of ozone pollution can therefore only be adequately dealt with as a result of international agreements.


Sulphur Dioxide (SO2)

There are no substantial sources of Sulphur Dioxide (S02) pollution within Hertsmere and it is not of concern. 


Where can I get further information

The HBAPMN aims to provide consistency in monitoring across the region and collates air quality monitoring results from across Hertfordshire and Bedfordshire. This data is available to the public on the network website listed below. The website gives air quality data for the council, as well as all local authorities in Hertfordshire and Bedfordshire. Another useful site that provides information and legislation on air quality is the Government site Department of Environment for Food and Rural Affairs, the Government body that deals with air quality. See below for the Defra website.

DEFRA website

HBAPMN website



All local authorities report to Defra regarding the progress they are achieving for air quality. See below for the most recent reports:

Updating and Screening Assessment 2006 (USA)

This report in June 2006 concludes that Hertsmere Borough Council is not required to carry out a Detailed Review and Assessment for carbon monoxide, benzene, 1,3 butadiene, lead, nitrogen dioxide, PM10 or sulphur dioxide.

Detailed Assessment 2007

This detailed assessment confirmed that the air quality objective for nitrogen dioxide will not be achieved at residential properties close to the Broadway at the junction of Darkes Lane, Mutton Lane and Baker Street. The report recommended that Hertsmere Borough Council declare this area as an Air Quality Management Area.

Air Quality Review and Assessment 2008

The Air Quality Review and Assessment 2008 highlight the need for further investigation in areas Southgate Road, Potters Bar and Park Road, Radlett for nitrogen dioxide.

Updating and Screening Assessment 2009 (USA)

This report proposes actions to undertake further monitoring of nitrogen dioxide at High Street Bushey and Watling Street / Aldenham Road junction Radlett. To carry out detailed assessments for Elstree Crossroads, Southgate Road / Barnet Road / High Street Potters Bar and junctions of Watling Street / Aldenham Road and Watling Street / Park Road Radlett, plus M1 Bushey at Hartspring Lane.

Detailed Assessment 2010

Following on from the USA 2009 the Detailed Assessment recommends that the existing AQMA at Potters Bar High Street is amended. The existing AQMA at Elstree Crossroads is extended.

Progress Report 2010

The proposed actions from this report recommend continuing monitoring of nitrogen dioxide at High Street Bushey and Watling Street / Aldenham Road Radlett. To complete the 2010 Detailed Assessment recommendations at Elstree Cross Roads, Barnet Road / Southgate Road / High Street and the High Street at the bus station. Radlett junctions of Watling Street / Aldenham Road and Watling Street / Park Road and Hartspring Lane M1

Progress Report 2011

The new monitoring data has not yet identified an immediate need to proceed to a Detailed Assessment for any pollutant. However the results for Shenley Road may need to undergo a Detailed Assessment after the 2012 USA.

Hertsmere USA 2012

The 2012 Updating and Screening Assessment considers all new monitoring data and assesses the data against the Air Quality Strategy objectives.  The need for Detailed Assessment for Nitrogen Dioxide at Shenley Road is confirmed.

Our Air Quality Action Plan Progress report

An action plan report is produced once an AQMA has been declared. This document is a report on the progress Hertsmere Borough Council has made on the 2003 Action Plan.

Progress Report 2014

The new monitoring data has not identified an immediate need to proceed to any further Detailed Assessments. It recommends that an outstanding Detailed Assessment is carried out for Shenley Road and Bushey High Street. 

Detailed Assessment for Shenley Road and Bushey High Street

as advised in Hertsmere USA 2012 report. The detailed assessment advises that Shenley Road is to be declared as an Air Quality Management Area. The report advises that Bushey High Street at the present time does not have to be declared.

Updating and Screening Assessment 2015

The 2015 Updating and Screening Assessment considers all new monitoring data and assesses the data against the Air Quality Strategy objectives.

Annual Status Report 2016

The Government from 2016 have changed the reporting system for Air Quality to the Department of Environment Food and Rural Affairs (DEFRA). The reports are simpler in format. This Annual Status Report (ASR) 2016 considers and assesses monitoring data from 2015. The report also highlights two new locations in Radlett and Borehamwood, which we will be considering as air quality management areas for nitrogen dioxide. These areas are Park Road and Watling Street in Radlett and Shenley Road in Borehamwood. Further information regarding these proposed air quality management areas can be found in the report.

Annual Status Report 2017

This Annual Status Report (ASR) 2017 considers and assess monitoring data from 2016. At the request of Department of Environment Food and Rural Affairs (DEFRA) distance calculations have been undertaken in this ASR 2017 report, whereas in previous years there have been no distance calculations. It is important to note this when comparing results between years, as the results for 2016 may appear to be lower.

Annual Status Report 2018

This Annual Status Report (ASR) 2018 considers and assess monitoring data from 2017. It concludes that concentrations of PM10 and PM2.5 were below the relevant air quality objectives in 2017 at all monitoring locations.  Nitrogen dioxide concentrations were, however, above the annual mean objective at one diffusion tube monitoring site (following distance correction) in 2017. This site (HM61) is located within AQMA 3 Blanche Lane, South Mimms, where concentrations have previously been measured above the objective. It is therefore recommended that AQMA 3 remains declared.  No exceedances of the 1-hour mean objective were measured at both the roadside and the background automatic monitoring stations. 

Annual Status Report 2019

This annual status report has been submitted to Defra and is awaiting approval. Once the report has been approved by Defra, it will published here.

Annual Status Reports 2020 & 2021

The latest air quality reports for our borough are in the process of being compiled and will be sent to DEFRA for approval, before being published on our website. Hertsmere Borough Council  are planning  to have all the ASR reports completed by 31 December 2021, depending on a number of factors which includes DEFRA’s approval. We would therefore advise you to keep referring back to our website for the latest reports as they become available. Apologies for any inconvenience this may cause you in the meantime.

Contact us 

If you need further information regarding air quality please contact Sarah Hoggett, Senior Environmental Health Officer on 020 8207 2277 or email