We already know pot doesn't cause lung or respiratory cancers when smoked:
http://www.washingtonpost.com/wp-dyn/co ... 01729.html
http://www.springerlink.com/content/l22 ... 52/?MUD=MP
We also know that marijuana smoking doesn't cause COPD:
http://www.ncbi.nlm.nih.gov/pubmed/19364790.1
And marijuana smoking's effects on the lungs, while they exist, are relatively mild and in no way comparable to tobacco's ravages:
http://vitals.msnbc.msn.com/_news/2012/ ... shows?lite
So basically, yeah, it's junk science. They know they can't prove that marijuana causes lung cancer or COPD(because it doesn't), so they try and publish as much scary-sounding but pointless evidence about cancer-causing chemicals and how bad marijuana smoke is for lab equipment or for cells in a petri dish. They're trying to scare people who don't know how to read the evidence, who don't understand for instance that the presence of a cancer-causing chemical in something doesn't mean that something is itself carcinogenic- coffee contains carcinogens for instance but its consumption is believed to reduce your risk of many cancers.
