Previous studies showed marijuana tar contained about 50 percent more of the chemicals linked to lung cancer, compared with tobacco tar, Tashkin said. In addition, smoking a marijuana joint deposits four times more tar in the lungs than smoking an equivalent amount of tobacco.
"Marijuana is packed more loosely than tobacco, so there's less filtration through the rod of the cigarette, so more particles will be inhaled," Tashkin said in a statement. "And marijuana smokers typically smoke differently than tobacco smokers -- they hold their breath about four times longer, allowing more time for extra fine particles to deposit in the lung."
He theorized that tetrahydrocannabinol, or THC, a chemical in marijuana smoke that produces its psychotropic effect, may encourage aging, damaged cells to die off before they become cancerous.