Particles

Short- and long-term exposure to particles causes deaths and increased admissions to hospital of people suffering from cardiovascular and respiratory diseases.

Particles found in the air are composed of a variety of materials including;

      • carbon
      • ammonium nitrate and ammonium sulphate
      • the oxides and salts of many metals
      • organic materials including rather large and complex molecules and fragments of soil. 

Some are emitted by vehicles and industrial sources, others come from dust thrown into the air from the surface of the ground and some are formed in the air by chemical reactions.

 The composition of the mixture varies from place to place: materials emitted by vehicles dominate the mixture near roads; in rural areas materials produced by chemical reactions in the air including nitrates and sulphates may dominate the mixture. 

The Air Quality Expert Group (AQEG) advises Defra on levels, sources and characteristics of air pollutants in the UK visit the  Defra website [external link]

 

Short- and long-term exposure to particles causes deaths and increased admissions to hospital of people suffering from cardiovascular and respiratory diseases.

Particles found in the air are composed of a variety of materials including;

      • carbon
      • ammonium nitrate and ammonium sulphate
      • the oxides and salts of many metals
      • organic materials including rather large and complex molecules and fragments of soil. 

Some are emitted by vehicles and industrial sources, others come from dust thrown into the air from the surface of the ground and some are formed in the air by chemical reactions.

 The composition of the mixture varies from place to place: materials emitted by vehicles dominate the mixture near roads; in rural areas materials produced by chemical reactions in the air including nitrates and sulphates may dominate the mixture. 

The Air Quality Expert Group (AQEG) advises Defra on levels, sources and characteristics of air pollutants in the UK visit the  Defra website [external link]

 

Particle size

 

Particles found in the air are classified by diameter and range in size from a few nanometres (nm) to several hundred micrometres (μm) in diameter.

PM10

      • PM10 refers to the mass concentration (expressed in micrograms per cubic metre μg m–3) of particulate matter that is generally less than 10 millionths of a metre (10 μm) in diameter (a micrometre is a thousandth of a millimetre).

PM2.5

      • The smallest constituents of PM10 are PM2.5, particles less than 2.5 micrometres across (often referred to as fine particles).

Coarse Fraction

      • The difference between PM10 and PM2.5 is known as the coarse fraction

Nanoparticles

      • The smallest elements of PM2.5 are the nanoparticles( also known as ultrafines ) less than 0.1 micrometres (=100 nanometres) in diameter. 

 

Particles of less than 10 µm diameter are important because this is the size range that can enter the lungs.  A fraction of the material will be deposited in the lungs and it is this which causes harm to health. 

Any particle less than 2.5 μm (including the nanoparticles) can deposit anywhere in the lung, from the upper airways down to the deepest sections where the uptake of oxygen occurs. Particles larger than 2.5 μm tend to deposit in the very upper parts of the airways that lead into the lungs.

Short-term exposure

 

Much of what is known of the effects of particles on health comes from epidemiological studies

Studies that have linked day to day variations in particle concentrations with effects on health occurring over a period of a day to few months are known as studies of the short term effects of exposure to particlesnurse_and_patient

The effects of short-term (or acute) exposure to particles include increases in hospital admissions and premature deaths in addition to less serious effects including a worsening of symptoms in those with respiratory disease and an increased number of GP consultations.  Evidence of restriction of daily activity has also been recorded. Read about epidemiological studies

It seems likely that those affected by respiratory and cardiovascular disease are at the greatest risk.  People suffering from diabetes have also been identified as a susceptible subgroup within the population.  Older people are likely to be at greatest risk. Read about Susceptible groups.

 Effects on the heart have been studied in detail.  It seems that heart attacks and sudden death, probably due to ventricular fibrillation, are the major causes of death associated with exposure to particles.  Whether these are due to changes in the clotting properties of the blood or to effects on the control of the heart rate remains to be seen but both explanations are possible.  It may be that very small particles play a part in these effects. Read about Cardiovascular disease and air pollution 

 

Quantifying the effects of short-term exposure to PM

 

The results (coefficients) of time-series studies allow estimates of the size or scale of the effects of air pollution to be calculated. Coefficients from time-series studies were used to help calculate the numbers of deaths and hospital admissions for respiratory or cardiovascular diseases associated with short-term exposure to particulate matter in COMEAP's 1998 report The Quantification of the Effects of Air Pollution on Health in the United Kingdom PDF, 3.13 MB'  (See table 1 below).

At that time COMEAP did not make any recommendations for quantifying effects due to long-term exposure.  Since 1998, the evidence on long-term exposure to particulate matter has grown.

COMEAP also recommended a coefficient for hospital admissions for cardiovascular disease associated with short-term exposure to particulate matter in its 2001 report. Read more about this in the cardiovascular disease and quantifying the health effects of air pollution sections.

 

Endpoint

Coefficient

Deaths (all causes)

 

 

Respiratory hospital admissions

 

 

Cardiovascular hospital  admissions

+0.75% per 10 µg/m3 increase in pollutant concentration (24 hour mean)

 

+0.8% per 10 µg/m increase in pollutant concentration  (24 hour mean)

 

+0.8% per 10 µg/m increase in pollutant concentration (24 hour mean)

 

Table 1 Estimates of coefficients to quantify short-term exposure to particulate matter PM10  COMEAP 1998, 2001

 

Long-term exposure

 

Long-term exposure to particles causes deaths from cardiovascular and respiratory diseases, including lung cancer

Cohort Studies deal with the relationship between long term concentrations of fine particles and risks of death. These studies have revealed that long-term exposure to particles causes deaths from cardiovascular and respiratory diseases.

Based on this evidence, the committee published its detailed estimate of the effect of particulate air pollution on the death rate in the UK in the follow up report in 2010: The Mortality Effects of Long-term exposure to particulate air pollution in the United Kingdom

This estimated that:

  • Each 1 µg/m3 reduction in annual average concentration of PM2.5, would increase life expectancy from birth by 20 days and the whole population (including new births) would in total live about four million years longer over the next 106 years.
  • Using 2008 figures, the burden of particulate air pollution from human activity (traffic, industry etc.) was estimated as equivalent to 29,000 deaths in 2008, associated with a loss of population survival time of 340,000 years or a loss of up to six months of life expectancy.

 

There are a number of uncertainties and simplifying assumptions when calculating the impact and burden of air pollution. The actual numbers could vary from about a sixth to double the figures shown.  Nevertheless, the numbers show that a reduction in particulate air pollution would have marked health benefits in terms of reduced death rates and the associated increase in life expectancy and in terms of the total years lived by the population as a whole.

Read more detail about how COMEAP has calculated the effects and what they mean in the quantifying the health effects of air pollution section.

 

Quantifying the effects of long-term exposure to PM on Mortality

 

The COMEAP 2009 report, Long-Term Exposure to Air Pollution: Effect on Mortality  concluded that the best estimate of the effects of long-term exposure to particulate matter was a 6% increase in risk of death from all causes being associated with a 10 μg m-3 increase in PM2.5 concentration. Estimates for increases in mortality from cardiopulmonary disease and lung cancer were also recommended.

 

Health Endpoint

Coefficient

 

All-cause mortality

 

 

 

 

 

 

1.06

95% confidence interval 1.02-1.11

 

 

 

 

For impact assessment of all cause mortality and assessing policy interventions designed to reduce levels of air pollutants

Use the full distribution of probabilities  see report for details

 

1.01 and1.12 as the 12.5th and 87.5th percentiles of the probability distribution

For sensitivity analysis

1.00 and 1.15  

 

For reports on quantification of risks from long-term exposure to particulate air pollution represented by PM2.5

Cardiopulmonary mortality

 

1.09 95% confidence interval 1.03-1.16

 

 

Lung cancer mortality

1.08 95% confidence interval 1.01-1.16

 

 

All coefficients expressed in terms of relative risk per 10  µg/m3 increase in PM2.5 (annual average concentration).

 

 Estimates of coefficients to quantify long-term exposure to particulate  matter PM 2.5 COMEAP 2009

 

 

 

 

 

 

 

 

 

 

 

 

   

Mechanisms of action

Composition

Particles are made up of many different chemicals, including metals and organic compounds, some of which generate free radicals and some researchers consider these to be involved in the toxicological effects of particles.

Airborne particles differ greatly from place to place in their sizes and chemical composition. 

Despite this, the effects of PM2.5 and PM10 appear to show considerable consistency between sites across the world ( Brook et al 2002).

Toxicity

There is no indication that just one of which particle properties, such as their size or the presence of specific chemical substances, are most responsible for their toxic effects. 

All the different size fractions of PM may result in biological effects. Currently there is more epidemiological evidence for fine particles, PM2.5.

The toxicity of PM may vary by its chemical composition. Components that may affect toxicity include:

      • Elemental and organic carbon
      • Secondary inorganic PM, such as sulphates and nitrates but effects are only seen at higher than ambient levels
      • Transition metals e.g. nickel, vanadium can produce free radicals (see below)
      • Organic constituents  such as polycyclic aromatic hydrocarbons that bind to particles and endotoxins, which are present in the coarse fraction
      • Primary PM, notably traffic emissions may be a significant contributor to adverse health outcomes

Read more - see WHO, 2006 and Health Effects Institute, 2008

Possible mechanisms

There has been a great deal of research into the possible mechanisms. It is likely that an inflammatory process due to oxidative stress (see glossary) caused by increased levels of reactive oxygen species (ROS) is responsible for many of the effects on cardiovascular and respiratory health. The mechanisms may include:

      • increased levels of clotting factors in the blood
      • effects upon heart rate variability,
      • Or possibly, a combination of a number of mechanisms. 

A fraction of those particles small enough to enter the lungs will be deposited on the surfaces of the upper airways and the alveoli (lower airways).  The lung has mechanisms for the removal of such particles.  Some particles, or components of particles, may however, react with the thin layer of fluid found on the surface of the airways and release compounds that set up an inflammatory reaction. 

Metals, for example, may do this by causing the production of oxidative free radicals.  Very small particles may enter the tissues of the lung where further inflammation may be produced.  This may have a knock-on effect on the cells lining the fine blood vessels of the lung and further signalling may occur.  Changes in levels of active clotting factors in the blood may follow.  Recent work by Künzli et al (2011) has shown that the thickening of arterial walls that occurs in atherosclerosis may be accelerated by exposure to fine particles.

In addition, particles deposited in the airways may interact with nerve endings and trigger reflex responses.  These include narrowing of the airways and, perhaps, changes in the rhythm of the heart. Read about the associations between exposure to particles and cardiovascular disease 

Carcinogenic compounds, including polycyclic aromatic hydrocarbon compounds may condense onto the surfaces of particles. Release of these substances in the lung, may explain the observed association between long-term average concentrations of fine particles and the risk of developing lung cancer.

 References mentioned above can be found in the further reading section