The toxicity of carbon particles ("particulate") has been stressed in the designation of PM10 and PM2.5, which refers to particles of size of 10 and 2.5 microns (thousandths of a millimetre) or less. The smallest of these particles are breathed into the deep lung, and during conditions where the concentration of them is high, an enhanced incidence of heart attacks and breathing problems is found. It is thought that the presence of the particles triggers the release of cytokines, which control various cellular responses, and this is the cause of such health problems during smogs.
The origin of the particles is the incomplete combustion of diesel fuel and though more tank to wheel miles are got from diesel than petrol, the emission of particulate poses a danger to health. By fine-tuning a diesel engine the amount of particulate formed can be minimised but rarely entirely eliminated. Burning biomass is a further significant source of carbon black.
Such carbon particles may also influence the health of the planet, and carbon black and CO2 cause the Earth to warm-up by different mechanisms. In the case of CO2, there is a contribution to the greenhouse effect, while particles of carbon black absorb some of the heat from sunlight directly and act like an atmospheric blanket that is becoming thicker as levels of pollution increases. Carbon black particles have a life-time in the air of typically just a few weeks, before they are removed by precipitation and gravity. Thus, if the sources of these particles were removed, the air would become clean of them fairly quickly, unlike CO2 which may hang around for centuries.
This is particularly significant for India and other developing countries in Asia, where a prominent mix of particles from burning biomass and fuels in vehicles arises, and India produces around 6% of the world total atmospheric budget of black carbon. Asian countries stress that it is Western nations that emit most of the world's atmospheric carbon and so should set an example in terms of curbing carbon emissions. However, since it is developing nations that emit relatively more black carbon per capita, they may be called to account and encouraged to limit those processes that are the origin of it.
It is significant that if a glacier becomes literally coated with a layer of carbon black, the extra absorbed heat will cause the ice to melt faster. Thus there is a particular link between carbon black and potential sea level rise. Black carbon is easier to curb than CO2 in that by reducing deforestation in which tropical rainforests are burned, and fitting diesel filters to vehicles a significant proportion of the particulate can be eliminated. Domestic stoves that burn wood and other biomass could also be replaced by cleaner alternatives. In addition to the amelioration of effects on climate, considerable improvements to the health of large populations of the world should be expected.
 "Black Carbon: An Overlooked Climate Factor." By Bryan Walsh: http://www.time.com/time/health/article/0,8599,1938379,00.html
"Toxicology of the Human Environment: the Critical Role of Free Radicals," Ed. Chris Rhodes. http://www.amazon.com/Toxicology-Human-Environment-Critical-radicals/dp/0748409165
ISBN-10: 0748409165; ISBN-13: 978-0748409167