En español | Winter weather is here, and with it comes a whole new set of health hazards we need to know about and protect against. Some threats are obvious — the heart attacks triggered by shoveling snow — while others are just as dangerous, but more subtle.

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Winter brings cold weather and a host of potential health dangers. — Getty Images

And the threats are from not just falling snow, but also falling temperatures.

Here are five hidden dangers you should know about:

1. Heart attack and stroke

Many winter heart attacks aren't from the sudden exertion of shoveling snow.

While the number of heart attacks does spike in the winter — by some estimates there are 53 percent more now than in the summer — that's true all across the country, including in some Sunbelt states that never see a snowflake. It's winter's cold, not just snow, that poses the threat. Our arteries respond to cold by constricting, and that makes us more prone to heart attacks. Why? Narrow arteries can cut down the flow of blood through the body, "making your heart work harder," says Roger Blumenthal, M.D., director of the Johns Hopkins Ciccarone Preventive Cardiology Center.

And even small temperature drops can cause big problems.

British researchers found that falling temperatures of less than 2 degrees Fahrenheit in a single day resulted in a 2 percent rise in the number of heart attacks that occurred during the next two weeks. That meant about 200 more heart attacks throughout England and Wales per "colder" day, according to a three-year study of temperature records and 84,000 hospital admissions.

Moreover, as we age, the cold hits us harder — especially when the thermometer drops to 32 degrees or below. "The older you get, the harder it is to regulate body temperature," says Ronan Factora, M.D., of the Cleveland Clinic's Center for Geriatric Medicine. "There's less fat and muscle, less ability to generate heat." So compared with those in their 50s, men and women in their 70s or 80s feel the cold more severely.

Constricting arteries, Blumenthal says, also can trigger tears or splits in the plaque that lines the walls of the arteries. When that happens, blood clots can form, triggering a heart attack or stroke, both of which also occur more frequently during winter.

2. High blood pressure

This cold-weather constriction of the arteries increases blood pressure. "Because there's less space for blood to flow, there's more resistance inside blood vessels," explains Factora.

3. Vitamin D deficiency

Getting too little vitamin D — the sunshine vitamin — during winter's gray days can be dangerous. Less sunlight means you tend to get less vitamin D because it's primarily absorbed through the skin. Low levels of D have been linked to an increased risk of osteoporosis, heart attacks, dementia, heart disease and Parkinson's disease. Several studies also have shown that people with low vitamin D levels were twice as likely to have a heart attack or stroke, compared with those with higher levels.

About 15 minutes of sun on arms each day is often enough to maintain the levels you need. Unfortunately, in some parts of the country, the sun virtually disappears for months. Some older people may need to take vitamin D supplements. Check vitamin D recommendations and talk to your doctor about taking a supplement.

Next: How to fight the winter blues.  »