MadSci Network: Medicine

Re: Is all smoke carcinogenic ?

Date: Sat Feb 26 16:37:43 2005
Posted By: Jens Peter Bork, M.D., Internal Medicine, Erlangen University Hospital
Area of science: Medicine
ID: 1104481924.Me

Hello Neil, 

thank you for your question regarding the carcinogenicity of smoke
compounds. I can provide two answers, a short and a long one. 

The short answer is: Yes, almost all combustible matter produces
carcinogens when burned. 
The long answer is altogether more complicated. For instance, what is
combustible matter? We know that almost anything can burn given sufficient
heat and oxygen supply, even things such as steel, glass, concrete, bricks,
limestone, metals.... Second, what is burning? - if we took this to mean
any oxidative process occurring at high temperatures I could just copy and
paste half a chemistry book here and still not cover everything. Since I
lack the expertise of a chemist, the long answer cannot be too specific. If
you are interested in specific substances resulting from specific
combustion conditions, consider resubmitting your question to one of the
Chemist Mad Scientists.

Let us restrict ourselves to organic matter as combustible and plain old
fire as the process we consider. Even then, the chemistry involved is
extremely complex. What you get from a given material during a combustion
process depends critically not only on the substance being burned
(first-class coal vs. wet wood from an old forest shack with a peel of old
paint on it vs. a piece of sirloin steak left on the barbecue
inadvertently), but also on temperature (super-efficient high temperature
burner in the latest environmentally-friendly household heater burning
natural gas vs. a wood fire lit by the boy scouts one drizzly November
night), and oxygen supply (a block of wood in a basement with no windows
vs. a scrub of underwood on a dry, stormy summer day in California).

If you imagine an ideal case (the supermodern household heater burning
extremely pure natural gas), it is possible to produce, in principle,
carcinogen-free "smoke": With complete combustion (high temperature high
oxygen availability), all hydrocarbons can be reduced to CO2 and H2O,
Carbon dioxide and water. CO2 is problematic because of its properties as a
greenhouse gas, but that is an entirely different story. Modern
industrial/household waste combustion facilities can do almost the same
(produce nearly toxin/carcinogen-free emissions from an array of substances
which are less than ideal in almost every respect.
Three problems exist with this picture. First, you do not normally burn
pure hydrocarbons; organic chemistry is complex, involving a lot more atoms
than simply hydrogen, carbon, and - in the case of fire - oxygen
(otherwise, life couldn't be based on it). Second, it is not technically
possible to burn everything in this kind of ideal high tech heater (think
of the block of wood from the basement); third, this kind of combustion is
extremely expensive and not very energy-efficient (high temperatures,
demand for precise steering of oxygen availability...). So it is the rule,
not the exception, that fires do not produce exclusively H2O and C2O; they
burn "dirty". And that is why almost any kind of smoke will contain some

A good example of burning „dirty“ is cigarette smoke; the temperature of a
burning cigarette is low, and oxygen supply is scarce (especially at the
inner edge of the burning part of the tobacco) and highly variable - it
rises when the smoker draws the smoke and drops again afterwards. The
burning is so inefficient that some compounds of the tobacco reach the lung
completely unchanged (among them is  nicotine - which is the whole point of
smoking, after all). There are a lot of carcinogens in tobacco smoke (some
1400 in all); some of them do not have anything to do with combustion – for
example, the tobacco plant enriches Polonium, a radioactive (and thus
carcinogenic) metal from the soil; some are specific to the burning of
tobacco leaves;  but a lot are simply in the smoke because organic material
is being burnt.

Three classes of chemicals occurring in combustion processes have received
a lot of attention because of their carcinogenic potential: The dioxins,
rating among the most toxic substances known to mankind; the nitrosamines,
some of which are also produced when meat is fried or smoked, but which
also occur in cigarette smoke; and finally, the polycyclic aromatic
hydrocarbons (PAHs); these contain phenolic ring structures and are thus
especially stable; very high burn temperatures are needed to break them
down. This class is especially relevant as a source of environmental
carcinogens because they are produced in the combustion of fuels,
especially wood and fossil fuels (coal, oil, gas). They also arise in the
combustion of household/industrial wastes when older, outdated combustion
facilities are used.

As I said, I can only address your question in very broad terms; here is
some further reading. All sources are available free on the internet: [1]
contains excellent overview about carcinogenesis in general, especially
Cancer-ET-MECH.ppt (PowerPoint presentation); [2] deals specifically with
the carcinogenicity of PAHs; [3] with the carcinogenicity of cigarette
smoke; and [4] presents the results of a study showing that the air in
restaurants, bars etc. does indeed become less carcinogenic after a smoking
ban. For more general information, you might turn to books on occupational
and environmental medicine which your University’s library should be well
equipped with. 

Sorry for the delay (tough subject, this one) and have fun exploring!

Jens Peter Bork

(Citation: Pure & Appl. Chern., Vol. 68, No. 2, pp. 301-308, 1996)-
(Citation: Journal of the National Cancer Institute, Vol. 91, No. 14,
1194-1210, July 21, 1999)

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