Neurological disease deaths seem to be on the rise, a new study by British researchers shows, and it’s not just because people are living longer than in the past.

The scientists examined World Health Organization data on total neurological deaths in 21 Western countries. The data reflected two decades, starting in 1989 and ending in 2010.

The study involved looking for trends in each country’s neurological disease death rates. In addition, the researchers also compared each nation’s neurological disease death rate to similar data on other diseases, namely the nation’s mortality rates for cancer and cardiovascular disease.

In keeping with other research, the analysis showed that mortality rates for cancer and cardiovascular disease actually decreased between the starting and ending time periods. This was true of the U.S. rates as well as the average rates for all countries studied.

But neurological disease death rates were another story entirely.

Among men ages 55 to 74, the average neurological death rate for all countries combined rose by about 2 percent, up to 503 per million people. For women, there was a 1 percent increase, bringing the average rate to 390 per million people.

Some countries… namely the United States… had it much worse. For men in this age range, the U.S. rate rose by 82 percent, the largest jump for this category.

The numbers were bleaker for those who are 75 and older. In this demographic, the study-wide averages more than doubled for both genders.

Why so much of an increase in death rates for neurological disease? The researchers’ evaluation did not point to longer life spans as a major cause. With so much growth, they said environmental factors, such as various forms of pollution, may be to blame.