Nitrogen oxides (NOx) are a group of gases that are predominantly formed during the combustion of fossil fuels. The majority of NOx emitted as a result of combustion is in the form of nitric oxide (NO). Once released to the atmosphere, NO rapidly reacts with other gases present in the air forming nitrogen dioxide (NO₂), which is harmful to health.

Nitrogen dioxide has a variety of environmental and health impacts. It is a respiratory irritant that can cause inflammation of the airways which may exacerbate asthma and lower resistance to respiratory infections.

NO₂ is also a key component in the formation of ground-level Ozone, another atmospheric pollutant. 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. NOx gases also contribute to climate change, through Ozone formation and aerosol effects.

In 2023, data from the National Atmospheric Emissions Inventory (NAEI) indicates the main sources of NOx are road transport (30%), energy generation, for example power stations and refineries (19%), domestic and industrial combustion (18%) and ‘other’ transport such as rail and shipping (15%). Data from the National

The Department for Environment, Food & Rural Affairs (Defra) estimates that 80% of NOx emissions in areas where the UK is exceeding NO₂ limits are due to transport, with the largest source being emissions from diesel light duty vehicles (cars and vans). Therefore, concentrations tend to be highest in urban environments with high traffic levels.

Sources indoors include heating and cooking with carbon-containing fuels (coal, coke, gas, kerosene and wood). NOx are also produced naturally by lightning, and by microbial processes in soils.

NO₂ is a key pollutant assessed within the LAQM framework and one of the five pollutants forming the UK Daily Air Quality Index and Pollution Forecasts.

Defra report 375 of the 400 declared AQMAs in England are a result of exceedances of NO₂. As such NO₂ is the most widely monitored pollutant across Worcestershire currently.

UK wide emission limits or ceilings for NOx are also set out in the National Emissions Ceilings Regulations 2018 to reduce long range transboundary air pollution.

Further information:

Look at the interactive maps of Current Air Quality to find out the current status of monitored pollutants in Worcestershire.

See the Local Air Quality Reporting section for details of monitoring and annual assessment of pollutants in the county and local authority plans for tackling high concentrations of NO₂.

Particulate Matter is a generic term used to describe complex airborne mixtures of solid particles and liquid droplets of varying size, shape, and composition. Some particles are emitted directly (primary PM); others are formed in the atmosphere through complex chemical reactions (secondary PM). The composition of PM varies greatly and depends on many factors, such as geographical location, emission sources and meteorology.

PM is generally classified according to aerodynamic size in microns (one-thousandth of a millimetre):

• coarse particles, PM₁₀ (particles that are between 10 and 2.5 microns (μm) in diameter)
• fine particles, PM₅ (particles that are less than 2.5 μm in diameter)
• ultrafine particles PM₁ (particles that are less than 0.1 μm in diameter)

PM is not a single pollutant; the composition of particles is in practice very complex, comprising variable amounts of organic and inorganic chemicals derived from direct emissions or from atmospheric processing. While the effect of the PM size classes PM₁₀ and PM_2.5 on human health are known, differences in contribution to human health effects from each of the chemical components remains uncertain.

The size of particles and the duration of exposure are key determinants of potential adverse health effects. Particles larger than 10 µm are mainly deposited in the nose or throat, whereas particles smaller than 10 µm are small enough to be inhaled and can travel into our airways. There, they can cause inflammation and worsen the condition of people with heart and lung diseases. The strongest evidence for effects on health is associated with fine particles (PM_2.5) as they can be carried deep into the lungs. These particles may carry surface-absorbed toxic, or carcinogenic, compounds into the body.

Long-term exposure to particulate matter is associated with premature death, especially from heart and lung conditions. Studies have also suggested that high levels of PM_2.5 in childhood can permanently impair lung function.

Both PM and the gases that can form it are capable of being transported over large distances, so impacts may occur far from the original source. Around 15% of UK PM comes from naturally occurring sources, up to a third from other European countries and around half from UK human-made sources (CAS 2019).

Natural sources include wind-blown soil and desert dust (e.g. from the Sahara), pollen, sea spray particles, fires involving burning vegetation and volcanic material (Primary PM). 

The main sources of man-made (primary) PM are combustion processes from road vehicles (diesel engines), emissions from industry, solid fuel burning from domestic properties and biomass combustion and from friction and abrasion, such as tyre and brake wear on road vehicles.

Secondary PM is formed in the atmosphere through chemical reactions between other air pollutant gases such as nitrogen oxides (NO_X), ammonia (NH₃) and sulphur dioxide (SO₂).

In 2023, data indicates 20% of UK PM_2.5 and 10% of PM₁₀ emissions come from burning wood and coal in domestic open fires and solid fuel stoves; 21% of PM_2.5 and 18% of PM₁₀ comes from road transport (e.g. fuel related emissions and tyre and brake wear) and a further 16% of PM_2.5 and 38% of PM₁₀ comes from solvent use and industrial processes (e.g. steel making, brick making, quarries, construction). Since 1990 annual emissions of PM₁₀ fell by 70%, and emissions of PM_2.5 fell by 76%.

Air Quality Objectives are in place for the protection of human health for PM₁₀ and PM_2.5. PM₁₀ is a key pollutant assessed within the LAQM framework.  The Government introduced new national targets for PM_2.5 in the Air Quality Strategy 2023. Both of these metrics are two of the five pollutants forming the UK Daily Air Quality Index and Pollution Forecasts.

UK wide emission limits or ceilings for PM_2.5 are also set out in the National Emissions Ceilings Regulations 2018 to reduce long range transboundary air pollution.

Further information:

Look at the interactive maps of Current Air Quality to find out the current status of monitored pollutants in Worcestershire.

For further information on the health impacts of particulate matter and other pollutants go to Air Pollution and Health.

Sulphur dioxide (SO₂) is produced when sulphur-containing fuels or material, such as coal, are burned. It is a corrosive, acidic gas which is harmful to health and combines with water vapour in the atmosphere to produce acid rain.

Both wet and dry deposition have been implicated in the damage and destruction of vegetation and in the degradation of soils, building materials and watercourses. SO₂ has an irritant effect on the lining of the nose, throat and airways, and the effects are often felt very quickly. It can exacerbate asthma and cause respiratory effects. It is associated with chronic bronchitis and also with cardiovascular effects.

Chemical reactions of SO₂ can also produce sulphates, which remain in the air as secondary particles, contributing to particulate matter pollution.

The largest anthropogenic source of SO₂ in the atmosphere is the burning and processing of coal in power stations for electricity generation, other industrial facilities and in shipping. Smaller sources of SO₂ emissions include industrial processes such as extracting metal from ore and vehicles using sulphur containing fuels. However, domestic coal burning can result in significant indoor exposure to SO₂. Natural sources such as volcanoes, also contribute substantial quantities of SO₂ to the atmosphere globally, sometimes at high altitude.

Sulphur in coal played a contributory key role in the health impacts of the London smog in 1952, where estimates of the resulting mortality range between 8,000 and 12,000 deaths. SO₂ emissions caused significant harm to forests and freshwater habitats in the Northern Hemisphere in the 1970s - 80s.

Following concerted action to reduce SO₂ emissions, such episodes no longer occur in the UK. Due to the increased use of natural gas and electricity, coal-burning is now relatively uncommon, and levels of SO2 have steadily declined in the UK since 1980.

SO₂ is a pollutant local authorities are required to consider within the LAQM framework and one of the five pollutants forming the UK Daily Air Quality Index and Pollution Forecasts.

Defra report only 1% of declared AQMAs in England are a result of exceedances of SO_2, mostly associated with proximal coal burning power stations. As no such facilities exist within Worcestershire SO₂ is not currently monitored across the county.

UK wide emission limits or ceilings for SO₂ are also set out in the National Emissions Ceilings Regulations 2018 to reduce long range transboundary air pollution.

Further information:

Look at the interactive UK Air Quality Map on the Current Air Quality page to find out the current status of Sulphur dioxide in the UK.

For further information on the health impacts of sulphur dioxide and other pollutants go to Air Pollution and Health.

Ozone (O₃) is a very reactive chemical, which is potentially toxic to both plants and animals. 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.

Ozone can cause inflammation of the respiratory tract, eyes, nose and throat, irritating the airways of the lungs, and increasing the symptoms of those suffering from asthma and lung diseases. It can also cause damage to crops.

Unlike other pollutants, ozone is not emitted directly. Rather, it is formed as a result of a complex series of reactions involving volatile organic compounds (VOC), sunlight and nitrogen dioxide (NO₂).

The involvement of sunlight in this process means that ozone levels in the UK tend to be highest in spring and summer. The reactions take time (hours or days) to generate ozone, and the highest concentrations are frequently experienced many miles away from the source of the precursor pollutant emissions. Thus a significant proportion of ozone incidents experienced in the UK are due to emissions of its precursor pollutants transported from abroad. The problem of ozone pollution can therefore only be adequately dealt with as a result of international agreements.

Whereas nitrogen dioxide acts as a source of ozone, nitric oxide (NO) destroys ozone and acts as a local sink (NO_X-titration). For this reason, O₃ concentrations are not as high in urban areas (where high levels of NO are emitted from vehicles) as in rural areas. Ambient concentrations are usually highest in rural areas, particularly in hot, still and sunny weather conditions which give rise to summer smog’s.

However, increases in suburban and urban O₃ are occurring due to the reductions in primary emissions of NOx (nitrogen oxides) – see Trends in Air Pollutants.

Air Quality Objectives for ozone are set out in the Air Quality Standards Regulations 2010 and is one of the of the five pollutants forming the UK Daily Air Quality Index and Pollution Forecasts. However O₃ is not a key pollutant local authorities are required to assess as part of the LAQM framework. 

Further Information

Look at the interactive maps of Current Air Quality to find out the current status of monitored pollutants in Worcestershire.

For further information on the health impacts of Ozone and other pollutants go to Air Pollution and Health.

Ammonia (NH₃) is a gas released into the atmosphere from natural and man-made sources. Once emitted into the atmosphere, the subsequent deposition of NH₃ can be a major source of pollution, causing nitrogen (N) enrichment (eutrophication) and acidification of soil and water sources. NH₃ stays in the atmosphere for just a few hours as a gas also reacts with acid gases, such as sulphuric and nitric acid, to form fine particles of ammonium compounds (secondary PM_2.5). In this form it can travel very long distances before being removed from the atmosphere by rain and snow and deposited to land.

In this way ammonia can cause significant long-term harm to sensitive habitats, depositing more nitrogen onto soils and plants, and into freshwaters, than they can cope with. This has led to significant changes to plant communities, and also affects the animal species that depend on them.

The main health impacts of NH₃ arise through its role in secondary PM_2.5 formation and health effects associated with exposure to PM, as described above. Agricultural emissions of NH₃ have been reported to be key contributor to some short-term episodes of high PM pollution in recent years.

Agriculture is the dominant source of NH3 emissions (85 – 90%). It is emitted during storage and spreading of manures, slurries and fertilisers. A further 4% of NH3 emissions come from the waste sector. Remaining ammonia emissions are from a diffuse mix of sources including vehicles, human waste and industry.

Emissions of ammonia fell by 13% between 1980 and 2015. However, since then there has been an increase in emissions, largely as a result of fertiliser use.

UK wide emission limits or ceilings for ammonia are set out in the National Emissions Ceilings Regulations 2018 to reduce long range transboundary air pollution. However NH₃ is not a key pollutant local authorities are required to assess as part of the LAQM framework. 

For further information on the health impacts of ammonia and other pollutants go to Air Pollution and Health.

VOCs are a very large group of organic chemicals which differ widely in their chemical composition and are volatile at room temperature. They are responsible for example for the odour of scents and perfumes as well as pollutants.

VOCs are emitted from a wide variety of products and processes, including industrial processes and agriculture, and they also form a significant component of indoor air pollution emitted from household products.

Sources of VOCs can be anthropogenic or biogenic. Anthropogenic sources include fossil fuel use (petrol vapour) and production; solvents used in coatings, paints, and inks and biomass combustion. Other sources of VOCs include furnishing, carpets, and upholstery, products for cleaning and polishing, air fresheners, and personal care products, for example fragrance, deodorants, and hair styling products. Thus indoor exposure can be significant.

Biogenic VOCs (BVOCs) encompass VOCs emitted by plants, animals, or microorganisms.

Most VOCs are not acutely toxic, but may have long-term chronic health effects

Anthropogenic VOCs are regulated by law, especially indoors, where concentrations are the highest. VOCs emitted from consumer products are not thought be to a significant public health issue indoors when homes are well ventilated and when the products are used according to the manufacturers’ instructions. But some people may suffer irritation of the upper airway system (eyes, nose and throat), respiratory effects, headaches and dizziness if they are exposed. A growing body of research links indoor air pollutants and carcinogenicity risk.

A particularly important non-methane VOC (NMVOC) is formaldehyde, which can be released from furniture, finishes and building materials, such as laminate flooring, kitchen cabinets and wood panels, and is also formed in chemical reactions in the air between other NMVOCs and chemicals generated from combustion processes, such as smoking, heating, cooking or candle burning. At low concentrations, exposure to formaldehyde can cause irritation to the eyes and upper airways, and is classified as a human carcinogen.

The two most widely monitored compounds are Benzene and 1,3-butadiene. Benzene is a VOC which is a minor constituent of petrol (about 2% by volume). The main sources of benzene in the atmosphere are the distribution and combustion of petrol. Of these, combustion by petrol vehicles is the single biggest source (70% of total emissions) whilst the refining, distribution and evaporation of petrol from vehicles accounts for approximately a further 10% of total emissions. Benzene is emitted in vehicle exhaust as unburnt fuel and also as a product of the decomposition of other aromatic compounds. Benzene is a known human carcinogen.

1,3-butadiene, like benzene, is a VOC emitted into the atmosphere principally from fuel combustion of petrol and diesel vehicles. It is also an important chemical in certain industrial processes.

In the outside atmosphere, VOCs also react with NOx in the presence of sunlight to form ground level ozone, known to be harmful to health and the environment. Chemical reactions also occur indoors though are different because ultraviolet light, the main driver of outdoor reactions, is absorbed by glass.

UK wide emission limits or ceilings for Non-methane VOCs (NMVOC) are set out in the National Emissions Ceilings Regulations 2018 to reduce long range transboundary air pollution.

Air Quality Objectives for Benzene and 1,3-butadiene are set out within the LAQM framework, however it is amongst a group of 4 pollutants Government does not expect local authorities to report on annually as objectives have been met for several years in the UK.

For further information on the health impacts of VOCs and other pollutants go to Air Pollution and Health.

Carbon monoxide (CO) is a colourless, odourless and tasteless gas, produced when fuels such as gas, oil, coal and wood burn without sufficient oxygen to complete the combustion process. These are sources of fuel used in many household appliances, including boilers, central heating systems, gas fires, water heaters, cookers and open fires. Burning charcoal, running cars and the smoke from cigarettes also produce CO gas.

CO interferes with the blood's ability to carry oxygen to the body's tissues and exposure to high indoor levels can be fatal, while exposure to lower levels can result in symptoms that resemble flu, viral infections or food poisoning.

Air Quality Objectives for CO are set out within the LAQM framework, however it is amongst a group of 4 pollutants Government does not expect local authorities to report on annually as objectives have been met for several years in the UK.

Lead was once a significant contributor to outdoor air pollution, and its health harms include neurological damage. Since the introduction of unleaded petrol in the UK there has been a significant reduction in urban lead levels, and it is no longer a major contributor to poor air quality.

In recent years industry, particularly secondary non-ferrous metal smelters, have become the most significant contributors to emissions of lead. The highest concentrations of lead and heavy metals are now therefore found around these installations in industrial areas.

In a domestic environment, lead exposure is most likely to come from paint or water pipes. Lead may also originate from contaminated soil, flaking external house paint, nearby industrial processes and roads, or through previous use of the land the property was constructed on.

Even small amounts of lead can be harmful, especially to infants and young children. In addition, lead taken in by the mother can interfere with the health of the unborn child. Exposure has also been linked to impaired mental function, visual-motor performance and neurological damage in children, and memory and attention span.

Heavy Metals (lead, arsenic, cadmium chromium, cobalt, copper, iron, manganese, nickel, selenium, vanadium and zinc) are emitted by metal processing, burning of waste and fossil fuels. These tend to be in small quantities and are measured in the PM10 fraction of air pollution.

Background concentrations of lead are 0.01 μg/m³ or less over most of the UK. There are some higher concentrations, particularly near industrial areas, but these are at concentrations well below the UK limit value of 0.5 μg/m³ annual mean.

Air Quality Objectives for lead are set out within the LAQM framework, however it is amongst a group of 4 pollutants Government does not expect local authorities to report on annually as objectives have been met for several years in the UK.

Further information on the regulation of Industrial and Permitted processes is provided in the Pollution pages of the website.

Polycyclic aromatic hydrocarbons (PAHs) are a large class of chemicals produced due to incomplete combustion of organic fuel sources. Sources include motor vehicles, biomass burning and wildfires. Most data on the health effects of PAHs is based on benzo[a]pyrene (BaP). Inhalation of BaP can cause lung cancer.

PAH are part of a group of other compounds collectively called Toxic Organic Micropollutants (TOMPS). They comprise a complex range of chemicals some of which, although they are emitted in very small quantities, are highly toxic or carcinogenic.

TOMPS can causing a wide range of effects, from cancer to reduced immunity to nervous system disorders and interfere with child development. There is no "threshold" dose - the tiniest amount can cause damage.