Health effects of black carbon

Warning - Black carbon can seriously damage your health. Photo: Esther Simpson / Creative Commons

Reducing people’s exposure to PM2.5 containing black carbon should lead to a reduction in the health effects associated with PM.

Exposure to black carbon is linked to health impacts such as cardiopulmonary morbidity and mortality, and reducing people’s exposure to particles containing black carbon will therefore also reduce such adverse health impacts, according to a recent report published by the World Health Organization (WHO).

Prepared for the Task Force on Health Aspects of Air Pollution under the Convention on Long-range Transboundary Air Pollution, the report was produced as input to the revision of the Convention’s Gothenburg Protocol (see article on front page), and it presents the results of a systematic review of evidence of the health effects of black carbon in ambient air.

The report concludes that toxicological studies suggest that black carbon may operate as a universal carrier of a wide variety of chemicals of varying toxicity to the human body, and that reducing people’s exposure to particulate matter containing black carbon should reduce its effects on their health.

Black carbon (BC) is said to be an operationally defined term, which describes carbon as measured by light absorption, and as such it is not the same as elemental carbon (EC), which is usually monitored with thermal-optical methods. As yet, there are no generally accepted standard methods to measure BC or EC in atmospheric aerosol, so there is a need for standardisation.

The main sources of black carbon emissions are diesel-driven combustion engines (in road vehicles, non-road mobile machinery and ships), residential burning of wood and coal, power stations using heavy oil or coal, field burning of agricultural wastes, as well as forest and vegetation fires.

Due to the location of these sources, the spatial variation of BC in ambient air is greater than that of PM2.5, but in general ambient measurements or model estimates of BC are said to reflect personal exposures reasonably well and with similar precision as for PM2.5.

The review was carried out by a number of experts selected by the WHO. After reviewing the available time-series studies, as well as information from panel studies, it was concluded that these provided sufficient evidence of an association of short-term (daily) variations in BC concentrations with short-term changes in health (all-cause and cardiovascular mortality, and cardiopulmonary hospital admissions). Furthermore that cohort studies provided sufficient evidence of associations of all-cause and cardiopulmonary mortality with long-term average BC exposure.

Studies of short-term health effects showed that the associations with BC are more robust than those with PM2.5 or PM10, suggesting that BC is a better indicator of harmful particulate substances from combustion sources (especially traffic) than undifferentiated PM mass. The evidence from long-term studies was however inconclusive – in one of the two available cohort studies using multi-pollutant models in the analysis, the effect estimates for BC were stronger than those for sulphates, while an opposite order in the strength of relationship was suggested in the other study.

According to the report, there are not enough clinical or toxicological studies to allow an evaluation of the qualitative differences between the health effects of exposure to BC or to PM mass (for example, different health outcomes), or to allow quantitative comparison of the strength of the associations or identification of any distinctive mechanism of BC effects.

The review of the results of all available toxicological studies suggested that BC (measured as EC) may not be a major directly toxic component of fine PM, but it may operate in particular, as a universal carrier of a wide variety of combustion-derived chemical constituents of varying toxicity to sensitive targets in the human body such as the lungs, the body’s major defence cells and possibly the systemic blood circulation.

Based on these findings, the Task Force on Health agreed that a reduction in exposure to PM2.5 containing BC and other combustion-related PM material for which BC is an indirect indicator should lead to a reduction in the health effects associated with PM. The Task Force therefore recommended that PM2.5 should continue to be used as the primary metric in quantifying human exposure to PM and the health effects of such exposure, and for predicting the benefits of exposure reduction measures. It also recommended that the use of BC as an additional indicator may be useful in evaluating local action aimed at reducing the population’s exposure to combustion PM.

Christer Ågren

Health effects of black carbon. By Nicole AH Janssen, Miriam E Gerlofs-Nijland, Timo Lanki, Raimo O Salonen, Flemming Cassee, Gerard Hoek, Paul Fischer, Bert Brunekreef and Michal Krzyzanowski. Available from WHO:

In this issue