On 2015 Oct 31, David Keller commented:

Marijuana smokers were compared with tobacco smokers who smoked 40 times more cigarettes per day

Pletcher and colleagues compare the long-term effects on pulmonary function of smoking marijuana versus smoking tobacco. They conclude that tobacco smokers suffer worse degrees of pulmonary function impairment, as measured by spirometry, than do marijuana smokers for comparable amounts of use of each substance. However, their formula for defining equivalent levels of tobacco versus marijuana smoking is heavily biased and seems designed to yield a comparison which is highly favorable to marijuana use.

In each defined level of use, the tobacco smokers at that level inhale far more hot smoke and combustion products per day than do the marijuana smokers to whom they are compared. For example, in Table 2, the heaviest category of tobacco smokers is defined as those with a lifetime smoking history exceeding 20 pack-years (146,000 cigarettes), while the highest category of marijuana smokers is defined as those with a history of greater than 10 joint-years (3650 marijuana joints). Thus, the tobacco user smokes 40 times as many cigarettes, over the same period of time, as a marijuana smoker with whom he is compared.

However, it seems far more reasonable to suppose that the damage due to smoking any dried plant is proportional to the amount of hot combustion products inhaled. Since each tobacco cigarette contains roughly the same weight (if not more) of combustible plant matter as each marijuana joint, it is completely unreasonable to compare the pulmonary effects of smoking 40 tobacco cigarettes per day with the effects of smoking just one marijuana joint (cigarette) per day. Further, it is dangerously misleading to imply, based on such a biased comparison, that smoking marijuana is in any way less damaging to the lungs than smoking tobacco. To convince me of that, the investigators should compare smokers of tobacco cigarettes with marijuana smokers who inhale the hot smoke and combustion products from an equal number of marijuana cigarettes per day.

The worst potential harm from smoking any burning dried plant matter is lung cancer, an outcome not reported or compared in this study. Furthermore, even if marijuana smokers do suffer less loss of pulmonary function than do tobacco smokers (at equal numbers of cigarettes per day), all smokers would be better off if they did not smoke at all, even if they continued to use tobacco or marijuana by some less harmful route of delivery; tobacco smokers should switch to the nicotine patch, gum or spray, and marijuana users can ingest it orally and obtain the same psychogenic effects.

On 2015 Oct 31, David Keller commented:

Marijuana smokers were compared with tobacco smokers who smoked 40 times more cigarettes per day

Pletcher and colleagues compare the long-term effects on pulmonary function of smoking marijuana versus smoking tobacco. They conclude that tobacco smokers suffer worse degrees of pulmonary function impairment, as measured by spirometry, than do marijuana smokers for comparable amounts of use of each substance. However, their formula for defining equivalent levels of tobacco versus marijuana smoking is heavily biased and seems designed to yield a comparison which is highly favorable to marijuana use.

In each defined level of use, the tobacco smokers at that level inhale far more hot smoke and combustion products per day than do the marijuana smokers to whom they are compared. For example, in Table 2, the heaviest category of tobacco smokers is defined as those with a lifetime smoking history exceeding 20 pack-years (146,000 cigarettes), while the highest category of marijuana smokers is defined as those with a history of greater than 10 joint-years (3650 marijuana joints). Thus, the tobacco user smokes 40 times as many cigarettes, over the same period of time, as a marijuana smoker with whom he is compared.

However, it seems far more reasonable to suppose that the damage due to smoking any dried plant is proportional to the amount of hot combustion products inhaled. Since each tobacco cigarette contains roughly the same weight (if not more) of combustible plant matter as each marijuana joint, it is completely unreasonable to compare the pulmonary effects of smoking 40 tobacco cigarettes per day with the effects of smoking just one marijuana joint (cigarette) per day. Further, it is dangerously misleading to imply, based on such a biased comparison, that smoking marijuana is in any way less damaging to the lungs than smoking tobacco. To convince me of that, the investigators should compare smokers of tobacco cigarettes with marijuana smokers who inhale the hot smoke and combustion products from an equal number of marijuana cigarettes per day.

The worst potential harm from smoking any burning dried plant matter is lung cancer, an outcome not reported or compared in this study. Furthermore, even if marijuana smokers do suffer less loss of pulmonary function than do tobacco smokers (at equal numbers of cigarettes per day), all smokers would be better off if they did not smoke at all, even if they continued to use tobacco or marijuana by some less harmful route of delivery; tobacco smokers should switch to the nicotine patch, gum or spray, and marijuana users can ingest it orally and obtain the same psychogenic effects.