That's my guess: good for cognition, bad for mood. Even long-term.
I can't say it's totally healthy, smoked especially. Snuff is dredged in other chemicals and flavoring agents (probably this mostly affects oral health, oral cancer etc, and doesn't get into the blood or brain too much). Snus is probably healthiest, least additives, closest to the way nature intended.
Still it will have radioactives, due to farming techniques:
RADIUM-226 AND POLONIUM-210 IN LEAF TOBACCO AND TOBACCO SOIL.
Contents of radium-226 and polonium-210 in leaf tobacco and tobacco-growing soils vary with the source. The differences may result from production locality, culture, and curing. The polonium seems to be not entirely derived from the radium; plants probably take it up from the soil or air.
I accessed one study suggesting an association between adolescent cognitive dysfunction and smokeless tobacco. But again, this is the stuff with lots of additives. And generally, no offense, but tobacco smokers tend to care less about their overall health, when malnutrition contributes to cognitive dysfunction. So you have to control for these factors across the population sample, if you want a true estimate.
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The good news is you're missing out on pyrolytics by chewing:
Study on tobacco components involved in the pyrolytic generation of selected smoke constituents.
The aim of this study was to investigate the contribution of various tobacco components to the generation of smoke constituents using a tobacco pyrolysis model. We analyzed the amounts of primary tobacco components (sugars, protein, polyphenols, alkaloids, organic acids, inorganics etc.) in flue-cured and burley tobacco leaves. Each of the components was added to the tobacco leaves at the 0.5-fold and 1.0-fold amount naturally present in the leaves. The treated tobacco samples were pyrolyzed at 800 degrees C in a nitrogen atmosphere with an infrared image furnace, and the selected smoke constituents (benzo[a]pyrene, hydrogen cyanide, carbonyl compounds, aromatic amines, volatile organic compounds and phenolics) were quantitatively analyzed by several methods, including high performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GC-MS). The contribution of each tobacco component to the generation of selected smoke constituents was estimated from a regression line determined by the three yields (no addition, 0.5-fold addition, and 1.0-fold addition). The results of this study can provide useful and comprehensive information on the relationship between tobacco components and selected smoke constituents during pyrolysis.
It's really a mess with how many additives they put in. Within the last 5 years they added one just to stop it from burning in case you forget and leave it sitting. Not sure this was necessary.
Something as simple as adding sugar can cause an increased production of chemicals in smoke:
Chemical-analytical studies of the mainstream smoke of research cigarettes with various sugar application levels revealed that most of the smoke constituents determined did not show any sugar-related changes in yields (per mg nicotine), while ten constituents were found to either increase (formaldehyde, acrolein, 2-butanone, isoprene, benzene, toluene, benzo[k]fluoranthene)
There's a whole book on pyrolytic compounds in tobacco smoke, basically my suspicion is these are to blame for the majority of cognitive dysfunction observed in your study, and generally in smokers. It plays an undeniable role in Cannabis too; its smoke has 118 carcinogens, its vapor just 2[1]. And keep in mind that not all pyrolytics are carcinogenic, so this figure of 118 chemicals represents just the tip of the iceberg. Cheers to the vape or edibles.