MadSci Network: Chemistry

Re: Why does a lit match cover smells ?

Date: Tue Apr 6 18:55:37 1999
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 922951099.Ch

I can not refer you to state-of-the-art scientific research on this one, I 
am afraid. I am not even going to make it easy for you to settle your 
wager. There are several possibilities, and any or all of them might play a 

Firstly, how sure of your facts are you? Have the members of your 
fraternity carried out an extensive survey to ensure that smells are indeed 
covered by a burning or burnt match? I do not think it could be done 
scientifically. The sense of smell is both amazingly sensitive, and 
extremely fickle and subjective. It is capable of detecting many substances 
in quantities that no scientific instrument could detect. But it is also 
capable of becoming more or less sensitive with mood and with emotional and 
physical state. Often smells seem to be processed subconsciously, without 
ever entering one's immediate awareness. To do a scientific test, you would 
need to devise a control experiment, and a 'double-blind' protocol. That is 
there would need to be some inoccuous procedure other than lighting a match 
that occurred in the bathroom, and neither experimenter nor experimental 
subject should be able to tell whether they had encountered the lit match 
procedure or the control procedure when the subject had entered the 
bathroom and reported on the smell. That is an extraordinarily tall order!

There is a very real possibility that the report that a lit match covers 
smells is an urban myth. The nature of the sense of smell is such that a 
myth like that would tend to be self-fulfilling. Perhaps it is propaganda 
put about by persecuted smokers, attempting to justify use of the bathroom 
for their vice, as they are progressively expelled from public places ;-)

If we suppose for the sake of argument that the report is real, then there 
are (at least) three ways that a burning match could achieve such an 

When a match is struck, the first thing to burn is the head. The head of a 
match contains a chemical cocktail that includes a lot of sulfur. In the 
initial burst of combustion products there is therefore a lot of sulfur 
dioxide. Sulfur dioxide is an extremely pungent substance, to which the 
smell receptors are extremely sensitive. But it also has a very efficient 
numbing effect on the sense of smell. You can smell a minute amount of 
sulfur dioxide, but when you have done so, you will not smell anything else 
for a while. Sulfur dioxide is a gaseous combustion product, so if this is 
the main factor no-one wins the bet. It is not flame (plasma) nor smoke 
(solid aerosol). You can easily test this one, because if it is the main 
factor, other flames, like a spirit stove, a cigarette lighter, or a candle 
will not mask smells nearly as effectively.

If other flames are equally as effective as a struck match, then there are 
possibilities that either the smoke or the flame might be responsible, but 
the flame is the more likely of the two. 

Most substances that cause smells -- particularly bathroom sorts of smells 
-- are organic materials, often but not necessarily containing sulfur. 
These materials are subject to oxidation, and oxidation usually produces 
products that are less odoriferous. A flame is a plasma that contains, 
among other things, a lot of hydroxyl (OH) free radicals. These free 
radicals are very efficient oxidizing agents: they will oxidize any of 
these smelly materials to produce water and another free radical. This 
second free radical will then rapidly react with atmospheric oxygen to make 
an oxidized product and regenerate the hydroxyl radical. Because they are 
recycled in this way, a relatively small number of hydroxyl radicals can 
oxidize a lot of other stuff. So it is possible that a lot of the smelly 
material might be oxidized in the flame.

Smoke is made up of a lot of tiny particles of tar suspended in the air. 
Because these particles are very small, they have a very large surface area 
relative to their total volume. Molecules of smelly substances in the air 
tend to stick very efficiently to these surfaces (adsorption), and so a lot 
of smelly material can be removed from the gas phase to the particle 
surfaces in this way. But if you were then to inhale the smoke you might or 
might not still get the smells.

So, to give you an informed opinion (but not an authoritative answer):

I think that none of the four possibilities I have mentioned should be 
ruled out. My ranking in order of likelihood is--

1. The effect is a myth
2. It is brought about by the numbing effect of sulfur dioxide
3. It is brought about by oxidation of smelly substances in the flame
4. It is brought about by adsorption of smelly substances onto the smoke

I think that the wager should be settled by donation of the stake money to 
a worthy local charity.

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