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Health effects of black carbon
General characteristics of black carbon
Black carbon (BC), also known as soot, is an air pollutant that is emitted during combustion reactions. It consists of primary particles with a diameter between 20 and 150 nanometres (nm). BC is thus a subcategory of PM2.5 – fine particles with a diameter of less than 2.5 microns (µm). In accordance with Directive 2008/50/EC, the presence of PM2.5 particles in ambient air must now be monitored. For the next decade, the Directive lays down air quality objectives of ever-increasing rigour. These objectives relate both to concentrations of PM2.5 and exposure of the population in urban areas.
BC particles are characterised by a very high capacity to absorb visible light. Pure BC particles are rarely observed in the atmosphere, because they have a tendency to clot and react with other compounds in the ambient air, such as organic carbon and sulphates. These bound forms are categorized as soot. The term ‘soot’ thus refers to a group of pollutants related to the incomplete combustion of fossil fuels and biomass. The main sources of BC in the Brussels-Capital Region are transport (especially diesel engines) and heating.
Soot was one of the first air polluting categories for which health effects were recognised. Its impact was evident during dramatic periods of carbon pollution such as the disastrous winter of 1952 in London, when nearly 4,000 people died in the space of five days. In 1956, the Clean Air Act was introduced, greatly reducing soot emissions in the UK. Other countries followed suit by taking similar initiatives. These efforts had a very positive impact on air quality, making research on soot less interesting. The subject was abandoned in major scientific studies and replaced by research on PM10 and PM2.5. As a result, BC has not been very well studied up to the present time.
Measurement of BC concentration in the Brussels Region
In the Brussels Region, the BC concentration in the ambient air has been measured using an aethalometer at the Woluwe (WOL1), Molenbeek (R001), Ixelles (R002) and Uccle (R012) measuring stations since July 2009, July 2010, June 2011 and February 2012 respectively.
The results show a difference of a factor 10 between the concentrations of BC and those of PM10 particles (particles with a diameter of less than 10 µm). This finding is however not systematic: there are periods of very high concentrations of PM10 and lower concentrations of BC or vice versa, depending on the origin of the particles.
The correlation factors between concentrations of BC and those of other substances are stronger when these substances also result from combustion processes (traffic and heating), e.g. NOx (> 0.9 for the daily values in 2011, which taken as a whole also gives a correlation factor of ten), and to a lesser extent CO and CO2 (~ 0.8 for the daily values in 2011). This would suggest that BC is a good tracer for the pollution emitted by the main polluting sectors in the Region, i.e. traffic and heating.
Health effects of BC
In general, fine particles can penetrate deep into the respiratory system. The particles with the smallest diameter can penetrate into the alveoli, causing hazardous substances to enter the bloodstream. It is assumed that there is no threshold below which exposure has no effect.
While there is disagreement about the direct toxicity of black carbon, this substance acts beyond doubt as a carrier for various toxic substances, the most commonly cited being polycyclic aromatic hydrocarbons and heavy metals.
Cardiovascular diseases and carcinogenic effects of fine particles and/or BC
Today it is still uncertain whether the health impact of exposure to BC differs qualitatively or quantitatively from the impact of fine particulate matter in general. The number of studies investigating the health effects of both BC and fine particles is still too limited.
- There is no difference between the effects of black carbon and those of PM2.5 on the cardiovascular system in general, i.e. cardiac arrhythmia and heart failure, often leading to death.
- In general, lung function may be reduced, especially in patients with a respiratory deficiency, such as asthma sufferers. Moreover inflammatory reactions may arise in the lungs and, by spreading, lead to degradation of the autonomous nervous system, which indirectly affects heart function. Especially in children, a high level of exposure to fine particulate matter can affect the development of the lungs. Respiratory diseases frequently occur: bronchitis, chronic cough, sinusitis and colds.
- There is disagreement about the mechanisms of cancer development. Nevertheless it is clear that there is a link between exposure to fine particles and the risk of developing cancer. Among the PM2.5 particles, soot from diesel engines has the worst carcinogenic effect: it contains a high concentration of BC particles which can adsorb significant quantities of other compounds, such as polycyclic aromatic hydrocarbons.
There are few studies that single out BC. Most studies only deal with PM2.5. Various links have been established between exposure to PM2.5 and lower life expectancy. For example Miller et al. (2007) found that an increase in PM2.5 of 10 µg/m³ may increase the risk of fatal cardiovascular disease in women by 76%.
Groups at risk
The population groups that are most affected by these health risks are:
- children: as their lungs and respiratory system are still developing, exposure to fine particles can cause severe functional disorder;
- older people as their cardiovascular system is especially vulnerable;
- asthmatics and people who suffer from respiratory diseases.