If the media were actuarial about drawing our attention to the causes of avoidable death, newspapers would be filled with diarrhoea, Aids and cigarettes every day. In reality we know this is an absurd idea. For those interested in the scale of our fascination with rarity, one piece of research looked at a period in 2002 and found that 8,571 people had to die from smoking to generate one story on the subject from the BBC, while there were three stories for every death from vCJD.

So you've probably heard that smoking may prevent Alzheimer's. It comes up in the papers, sometimes to say it is true, sometimes to say it has been refuted. Maybe you think it's a mixed bag, that "experts are divided". Perhaps you smoke, and joke about how it will stop you losing your marbles.

This month, Janine Cataldo and colleagues publish a systematic review on the subject, but with a very interesting twist. First they found all the papers ever published on smoking and Alzheimer's, using an explicit search strategy which they describe properly in the paper – because they are scientists, not homeopaths – to make sure that they found all of the evidence, rather than just the studies they already knew about, or the ones which flattered their preconceptions.

They found 43 in total, and overall, smoking significantly increases your risk of Alzheimer's. But they went further. Eleven of the studies were written by people with affiliations to the tobacco industry. This wasn't always declared, so to double check, the researchers searched on the University of California's Legacy Tobacco Documents Library, a vast collection of scanned material which has been gathered over decades of legal action.

If you ever want to spend a chilling afternoon in the head of an industry whose product has been proven to kill a third of its customers, this is the place for you. "The importance of younger adults" uses financial modelling to explain the importance of recruiting teenage smokers to replace the dying older ones before it's too late, and explains that "repeated government studies have shown less than one third of smokers start after age 18 [and] only 5% of smokers start after age 24." "Youth cigarette – new concepts" from Marketing Innovations Inc takes these ideas further, into cola and apple flavour cigarettes, because "apples connote goodness and freshness".

How much did it matter if the researchers worked for the tobacco companies? A lot: the risks of Alzheimer's associated with smoking reported by these papers were on average about a third lower than those conducted by others, and they produced many papers showing cigarettes were protective. If you exclude these 11 papers, and look only at the remainder, your chances of getting Alzheimer's are vastly higher: comparing a smoker against a non-smoker, the odds are higher by 1.72 to 1.

So does that mean we can ignore all research that comes from people who disgust us? In Nazi Germany two researchers, Schairer and Schöniger, worked on biological theories of degenerate behaviour under Professor Karl Astel, who helped organise the operation that murdered 200,000 mentally and physically disabled people.

In 1943 the researchers published a well-conducted study demonstrating a relationship between smoking and lung cancer. Their paper wasn't mentioned in the classic Doll and Bradford Hill paper of 1950, it was referred to only four times in the 60s, once in the 70s, and then not again until 1988, despite providing a valuable early warning on a killer that would cause 100 million early deaths in the 20th century. It's not obvious what you do with evidence from untrustworthy sources, but it's always worth appraising its untrustworthiness with the best tools available.