It's not skipping a hat in winter or going outside with wet hair that may up your chances of pneumonia. Apparently, it's your vitamin D levels. A new study from Finland suggests that low vitamin D levels are a risk factor for the illness. Researchers mentioned nothing about headwear.

In 2009, 1.1 million people in the United States were hospitalized with pneumonia and more than 50,000 people died from the disease, reports the Center for Disease Control. Pneumonia kills more than 1.5 million children younger than 5 years of age each year around the world, a higher death toll than any other infectious disease, such as AIDS, malaria or tuberculosis.

For their study, University of Eastern Finland researchers looked at records for 1,421 subjects. The mean age of the subjects was 62.5 years. They compared the serum vitamin D3 levels of the samples drawn in 1998-2001 and compared them against reported cases of pneumonia in hospital records in the same people between 1998 and 2009. They found that people with the lowest D3 levels were more than 2.5 times more likely to contract pneumonia than the people with the highest levels of the vitamin. Smoking and age also increased the risk level. Men were more likely to get sick than women.

Earlier research has shown that vitamin D deficiency weakens the immune system and increases the risk of mild respiratory infections, according to the University's release. This new study is the first one to establish that vitamin D deficiency also increases the risk of contracting pneumonia in the aging general population. According to the researchers, this finding supports earlier observations on the diverse role of vitamin D in the body, and it also calls for further research on vitamin D deficiency as a public health issue.

The research was published in the Journal of Epidemiology and Community Health.