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Black Soot, Diesel Exhaust, and Indoor Air Quality
Written By: Henry P. Shotwell, Ph.D., CIH and Robert E. Sheriff, MS, CIH, CSP
February 24, 2020
Black Soot: What Is It?
What do diesel exhaust particles, black soot, carbon black and diamonds have in common? Answer: They are all forms of the element, Carbon! While diamonds may be a girl’s best friend, diesel exhaust particles and black soot surely aren’t. It’s the difference between black (Carbon) and white (Carbon)!!!
Diesel Emissions – Now A Suspect Cancer Agent
We’ve all seen black smoke belching from big rigs on the highway. Some of that exhaust is gaseous: oxides of nitrogen, mostly. The black “smoke” is made up of incompletely combusted carbon particles derived from the carbon in diesel fuel. Because diesel fuel is produced from crude petroleum oil, many of the carcinogenic components of crude petroleum, called Polynuclear Aromatic Hydrocarbons (often referred to as PNA’s or PAHs), are also exhausted along with the unburned carbon. Animal studies suggest that long-term exposure to very high doses of carbon black may increase the risk of developing cancer, but there is no clear-cut relationship just yet. To be prudent, we must assume the worst—that it causes cancer. OSHA has set a limit for worker exposure to carbon black.
Black Soot In Buildings
There is another phenomenon involving “soot.” Since the early 1990s, office workers, homeowners and apartment residents have complained about streaks of soot on carpeting at the thresholds of doors, at wall to floor joints and at other points where air passes from one region to another. The soot will typically outline items, such as pictures on a wall, the edges of draperies, and cabinets.
Fires and Tires – Much Black Soot
Three forces are necessary to create these streaks. Anyone or combination will do it: Impaction (forced air), Gravity and Attraction (electrostatic forces and moisture). Surprisingly, studies show that the major source of the carbon soot is soft, aromatic candle burning! Some candles had been shown to produce as much as 100 times as much soot as low-soot, hard wax candles. In offices, it is most likely from diesel vehicles, especially in commercial areas. Of course, any source of combustion products, such as gas-fired fireplace logs, wood fires, cigarette smoke, cooking by-products, boilers, and hot water heaters can generate soot.
Another source of black soot especially in urban areas and buildings near highways and interstates is tire rubber. If there is an unbalanced ventilation system or a significant temperature differential within or between rooms, soot deposition can occur. The best and obvious solution is to get rid of the soot producer or modify it to reduce or eliminate the soot. Balancing the air pressure between and within rooms will eliminate impaction as a causative factor.
Testing of Black Soot Can Now Identify It’s Source
It is possible to sample the black soot on surfaces or in the air to determine if it is carbon black, tire dust, elemental carbon from diesel exhaust. Another way to test for diesel exhaust is by testing/sampling the air for oxides of nitrogen.
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