Deaths from Alzheimer's disease have doubled over the past 30 years, and Alzheimer's-related mortality will probably continue to increase as people live longer. No prevention strategies or definitive treatments have been found, however, although the disease has been studied for decades.

A team of researchers from Harvard Medical School, Massachusetts General Hospital and Brigham and Women's Hospital recently looked at the association between death from Alzheimer's disease and a person's occupation across 443 occupations. They found that taxi and ambulance drivers had the lowest rate of death from Alzheimer's disease: 1.03 percent for taxi drivers and .91 percent for ambulance drivers.

Driving a taxi or an ambulance require daily, often unpredictable, navigation in real time, such as figuring out a taxi route or finding the fastest way to the hospital.

The study was motivated by a finding that showed that London taxi drivers had enlarged hippocampi. The research suggests that jobs requiring spatial processing may be associated with a lower risk of Alzheimer's disease mortality.

“Our results highlight the possibility that neurological changes in the hippocampus or elsewhere among taxi or ambulance drivers might account for the lower rates of Alzheimer's disease,” Anupam Jena, senior author on the study and a physician at Massachusetts General Hospital, said in a statement.

The way taxi and ambulance drivers navigate seems to make the difference. Driving a taxi or an ambulance require daily, often unpredictable, navigation in real time, such as figuring out a taxi route or determining the fastest way to the hospital. These tasks might cause changes to the hippocampus, the region of the brain used for spatial processing that is also involved in the development of Alzheimer's disease.

Other transportation-based professions — airline pilots, ship captains and bus drivers — did not have the same lower rate of Alzheimer's disease. The researchers believe this may be because they rely less on real time navigation and more on predetermined routes. As a result, they do not appear to have changes to the hippocampus that may reduce their risk of Alzheimer's disease.

Data for the study regarding subjects' occupation and cause of death came from death certificates in the National Vital Statistics System in the U.S. Of the nearly nine million people whose occupation was listed on their death certificate, almost 350,000, or 3.88 percent, had died of Alzheimer's disease.

This was an observational study, not a controlled experiment, and so cannot prove that taxi and ambulance driving causes changes to the brain that protect against Alzheimer's. More studies are needed to determine if the spatial and navigational processing skills required of taxi and ambulance drivers reduce their risk of Alzheimer's disease or Alzheimer's disease mortality.

The study is published in The BMJ.