emailed reply to the New Scientist magazine: 

"I was interested to read your article "Cannabis smoke beats tobacco for
toxic chemicals" (p9, 22/29 Dec 2007 issue.) 

This study makes it all the more intriguing to consider why around half of
all regular tobacco smokers will die from the habit (1), whereas no-one
has ever died from smoking cannabis (2). One possible explanation is their
differing action on the lungs. 

Tobacco smoke has a has a bronchoconstrictor effect on the lungs (3),
which traps dust, ash and tar. Cannabis smoke has a bronchodilator effect
on the lungs (4), and indeed has been has been known to stop a full blown
asthma attack (5). This effect promotes the release of dust, ash and tar
from the lungs. 

Some observers will point to the greater quantities and frequency
associated with tobacco smoking, but it seems possible that the differing
effects tobacco and cannabis have on bronchial diameter may be the
explanation for the very different results observed amongst their
respective user populations. 
:)
-- 
Brian Milner 
[Contact address redacted]

(1) http://info.cancerresearchuk.org/cancerstats/causes/lifestyle/tobacco/
"Around half of all regular smokers will die from the habit"
(2) http://www.ccguide.org.uk/young88.php 
"no credible medical reports to suggest that consuming marijuana has
caused a single death."
(3) http://jap.physiology.org/cgi/content/full/81/5/2053 
"Inhilation of cigarette smoke has repeatedly been shown to induce acute
bronchoconstriction in various species including humans" 
(4)
http://www.healthinparliament.org.uk/lungd/CANNABIS%20BRIEFING%20updated%20July%202006.pdf 
"The THC in cannabis has been shown to have a bronchodilator effect"
(5) http://www.medicalcannabisclub.ca/about.htm 
"Dr. Donald Tashkin of UCLA has stated that ingestion .of marijuana has
been known to stop a full blown asthma attack.