Air quality guidelines and health effects

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This page describes how air quality is assessed against both short and long-term objectives in the UK

Short-term:

The Daily Air Quality Index

The reference stations and Breathe London Nodes on the homepage map are coloured according to the government’s ‘Daily Air Quality Index’ (DAQI).

This index sets short-term thresholds for each of the four main pollutants measured in the UK and grades current air quality into four ‘bands’; Low, Moderate, High and Very High. Each of these bands has separate ‘index’ levels to provide gradation between the bands. The index runs on a scale from 1-10.

Where a monitoring station or Node measures more than one pollutant, the highest current index of any pollutant is shown.

The averaging periods and concentration breakpoints for each pollutant in the index is shown in the table below.

PM10 particulates PM2.5 particulates Nitrogen dioxide Ozone Sulphur dioxide
Band Index 24 hour mean (μg/m³) 24 hour mean (μg/m³) 1 hour mean (μg/m³) 8 hour mean (μg/m³) 15 minute mean (μg/m³)
Low 1 0-16 0-11 0-66 0-26 0-88
2 17-33 12-23 67-133 27-53 89-176
3 34-50 24-35 134-200 54-80 177-265
Moderate 4 51-58 36-41 201-267 81-107 266-354
5 59-66 42-46 268-334 108-134 355-442
6 67-75 47-53 335-400 135-160 443-531
High 7 76-83 54-58 401-467 161-187 532-708
8 84-91 59-64 468-534 188-213 709-886
9 92-100 65-70 535-600 214-240 887-1063
Very High 10 101 > 71 > 601 > 241 > 1064 >
 

The DAQI also provides health advice associated with each of the banding levels. These are shown below.

Banding Index General population At-risk individuals*
Low 1-3 Enjoy your usual outdoor activities Enjoy your usual outdoor activities
Moderate 4-6 Enjoy your usual outdoor activities Adults and children with lung problems, and adults with heart problems   who experience symptoms should consider reducing strenuous physical activity,   particularly outdoors.
High 7-9 Anyone experiencing discomfort such as sore eyes, cough or sore throat   should consider reducing activity, particularly outdoors. Adults and children with lung problems, and adults with heart problems   should reduce strenuous physical exertion, particularly outdoors, and   particularly if they experience symptoms. People with asthma may find they   need to use their reliever inhaler more often. Older people should also   reduce physical exertion.
Very High 10 Reduce physical exertion, particularly outdoors, especially if you   experience symptoms such as cough or sore throat. Adults and children with lung problems and older people, should avoid   strenuous physical activity. People with asthma may find they need to use   their reliever more often.
* Adults and children with heart or lung problems are at greater risk of   symptoms. Follow your doctor's advice about exercising and managing your   condition.
 

Strictly, the DAQI should only be applied to reference instruments. After deliberation, we have decided to apply the index to the Breathe London Nodes to provide essential health context but to also highlight the current network accuracy of the Breathe London Nodes vs the reference network.

Long-term:

National Air Quality Objectives

The National Air Quality Strategy sets out the UK’s air quality objectives which contain the legally binding limit values that should be attained by the objective date.

These are shown in the National air quality objectives and European Directive limit and target values for the protection of human health table below.

Pollutant Applies Objective Measured as Date to be achieved by (and maintained thereafter) European obligations Date to be achieved by (and maintained thereafter)
PM10 particulates UK 50 μg/m³ not to be exceeded more than 35 times a year 24 hour mean 31 December 2004 50 μg/m³ not to be exceeded more than 35 times a year 01 January 2005
UK 40 μg/m³ annual mean 31 December 2004 40 μg/m³ 01 January 2005
Scotland 50 μg/m³ not to be exceeded more than 7 times a year 24 hour mean 31 December 2010 50 μg/m³ not to be exceeded more than 35 times a year 01 January 2005
Scotland 18 μg/m³ annual mean 31 December 2010 40 μg/m³ 01 January 2005
PM2.5 particulates UK 20 μg/m³ annual mean 01 January 2020 Stage 2 Limit - 20 μg/m³ 01 January 2020
Scotland 10 μg/m³ annual mean 31 December 2020 Limit value - 25 μg/m³ 01 January 2015
UK urban areas Target of 15% exposure reduction at urban background annual mean Between 2010 and 2020 Target of 20% reduction at urban background Between 2010 and 2020
Nitrogen dioxide UK 200 μg/m³ not to be exceeded more than 18 times a year 1 hour mean 31 December 2005 200 μg/m³ not to be exceeded more than 18 times a year 01 January 2010
UK 40 μg/m³ annual mean 31 December 2005 40 μg/m³ 01 January 2010
Ozone UK 100 μg/m³ not to be exceeded more than 10 times a year 8 hour mean 31 December 2005 Target of 120 μg/m³ not to be exceeded more than 25 times a year averaged over 3 years 31 December 2010
Sulphur Dioxide UK 266 μg/m³ not to be exceeded more than 35 times a year 15 minute mean 31 December 2005 - -
UK 350 μg/m³ not to be exceeded more than 24 times a year 1 hour mean 31 December 2004 350 μg/m³ not to be exceeded more than 24 times a year 01 January 2005
UK 125 μg/m³ not to be exceeded more than 3 times a year 24 hour mean 31 December 2004 125 μg/m³ not to be exceeded more than 3 times a year 01 January 2005

Unlike the DAQI which is graded, assessment against the objectives is pass or fail. The objectives are legally binding.

Breathe London Nodes will be assessed against these objectives for each full year of data, with the indicative sensor accuracy caveat.

World Health Organisation guidelines

Around the world, nations set their own air quality targets and objectives. However, the World Health Organisation (WHO) also takes a role in setting global air quality guidelines to provide policymakers and governments with targets based on the latest scientific understanding and health research.

In September 2021 WHO updated their guidelines for the first time since 2005. Guidelines for PM10, PM2.5, and nitrogen dioxide were substantially reduced from their previous levels. The table below shows the current UK limits against the WHO guidelines.

Pollutant UK limit value (μg/m³) WHO guideline (μg/m³) Averaging period
PM10 particulates 40 15 Annual
PM2.5 particulates 25 5 Annual
Nitrogen Dioxide 40 10 Annual
Ozone 100 100 8 hour mean

On annual graphs and statistics, reference to these WHO guidelines is also provided for reference.