Very cold temperatures, like very hot ones, can be hazardous to your health. Proper dress and some sensible practices can prevent a lot of the problems associated with cold weather. In addition knowing the symptoms of danger and how to treat them can keep problems that do occur from becoming disasters.

COLD WEATHER FACTS:

• Hypothermia does NOT require temperatures below freezing (it can be much warmer than that), and it kills an estimated 28,000 people per year.
• Depending on how cold the temperatures (or Wind Chills) are, frostbite can set in mere moments, causing more than 50,000 amputations per year.
• Cold weather hazards are present across all 50 states, and hypothermia is possible in all seasons.
• The most succeptible to cold weather hazards include children and the elderly, the homless, and those who work consistently outdoors.

POTENTIAL HAZARDS FROM EXTREME COLD

• The most common hazard in the cold is frostbite. Your body doesn’t get enough heat and the body tissues freeze. Body parts most often affected by frostbite are the nose, ears, cheeks, fingers, and toes. Frostbite can cause permanent tissue damage, and require amputation.
• Hypothermia is another obvious hazard. It occurs after too much exposure to cold cools your body temperatures to dangerously cold levels. Since the average body temperature is around 98 degrees Fahrenheit, you can pretty much bet that hanging out in 80 degree water can do the trick (note: You're body doesn't do well at temperatures like 90, much less 80).
• Attempts to keep warm quickly often can get short-sighted and sloppy. It is no small wonder that the majority of house-fires and such in America occur in the dead of Winter. Serious attention needs to be paid to getting (and staying) warm safely.
• Any of the above hazards are exacerbated with smoking, drinking, medication, allergies, poor circulation, etc.

COLD WEATHER SAFETY

The NATIONAL WEATHER SERVICE will issue a "Wind Chill Advisory"  for an area if the Wind Chill temperature is expected to drop to around -15F and -24F. This Wind Chill threshold is set locally for an area, but in Southern New England, it's around this level. Keep in mind that the Wind Chill is an accurate reflection of the true cold experience on the human body, so the actual air temperature is not as much a factor, for if the winds were calm and the air temperature were to approach -15F, similar conditions would occur.

The NATIONAL WEATHER SERVICE will issue a "Wind Chill Warning"  for an area if conditions are expected that would bring the Wind Chill Temperature to -25F or below. Obviously temperatures colder than this are expected to be increasingly hazardous, but at this point, frostbite can occur in less than 15 minutes, so there's no point venturing out anywhere beyond this point without either absolute necessity or completeness of preparation.

Sometimes the NATIONAL WEATHER SERVICE will issue "Cold Advisories" or "Excessive Cold Advisories" or "Excessive Cold Warnings", etc. For areas that normally experience much warmer temperatures. This is also established locally, and would refer to temperatures that, though the won't reach the normal Wind Chill thresholds, hazardous conditions would certainly ensue for an area that is not expected to be normally prepared for such weather (think the 30's in Miami, etc.)

BEFORE ENCOUNTERING COLD WEATHER:

• Be Aware of the Weather Conditions - Make sure you know what the temperatures are outside before heading out. Something as simple as an outdoor thermometer that's readable from the inside can make a world of difference in being properly prepared. Know how long the cold weather is supposed to be in town, or when it will first strike. This is especially important if you are expecting to be outside for an extended period of time 
• Always Dress in Layers - In cold weather, having that big parka is not going to be as helpful as more layers of thinner clothing. Air is a great insulator, and if you can wear several layers there will be some air inbetween those layers, effectively multiplying your warmth retension.
• Wind and Moisture Protection is Essential - Nothing speeds up hypothermia than being wet, and nothing hastens the onset of frostbite than wind. The core of your body may feel nice and warm, but the extremeties are the first to experience frostbite. In the most dangerous of Wind Chill conditions, the entire body (head-to-toe) would need to be covered by wind-repellent clothing. Also, water-proof clothing is critical if outside in Winter snow, as you are interacting with water all the time. However, the Summer months pose dangers too, whether it be through swimming or hiking, or any other activity that puts you in contact with water (or sweat). Remember, when climbing uphill, a warm start with lots of sweat can become dangerous in higher elevations with cold winds and sweat on the body that won't evaporate.
• It Starts With the Head - The top of the body is where most of your heat would escape. Even if you are in many layers, the absence of a warm head-covering can effectively negate all sorts of preparation.

WHEN OUTSIDE IN THE COLD:

• Got Wet? It's Time to Go - This is one of the worst things that can happen in extreme cold (which is a hazard because you're often around snow/ice in cold weather). If you get wet, it's important to dry off immediately and/or put on dry clothing. If you're going to be out for an extended period of time, bringing along dry clothing (sealed in a container that would keep it so) is very important. Being wet out in the cold is just a silent plea for hypothermia to set in. 
• Keep Moving In the Cold - Physical exertion, especially in cold weather, will keep your body circulating and keep your internal engine going. Idling your body only increases the opportunity for coldness to creep in. This is true especially for hikers, but generally, it is not advised to stop moving in cold weather unless you are absolutely sure about your shelter, your clothing, AND your situation. Understand what different National Weather Service advisories mean, and what you could be expecting for snowfall.
• Know Where the Warmth Is - Any time you are outside in cold, or extreme cold, always be aware of where you need to go to be in shelter from the cold, and how to get there. Never hike without knowing the way (or knowing spots of shelter from the cold), and if you are aware of stretches in which shelter is not available, consider postponing your plans entirely.

HYPOTHERMIA:

• Cooling of the Body's Internal Temperature - When surrounded by cold air, your body may not be able to keep up with the cooling to maintain its normal temperature. Being wet when cold is the biggest precursor to hypothermia (in any season).
• Know the Signs - When hypothermia sets in, it's usually an internal thing for the body, however, there are some outward signs that someone is being affected internally by extreme cold. If someone starts to slow down, mumble and stumble, fumble objects, or simply become delerious or irrational, then those are definite signs of hypothermia.
• Seek Warmth / Dryness Immediately - It is critical that a person showing any of these signs be removed from the cold air as soon as possible. Removal of wet clothing is equally essential. To warm someone's core, a hot tea or jello is a great idea. Do not attempt to heat someone up too quickly (that type of shock can knock someone out). In really bad conditions, the only good option may be for one person to get in a sleeping bag with the other and personally share warmth.
• Seek Medical Attention - This would go without saying, but it's important, even if you think someone has been effectively 'saved' from hypothermia. You need to make sure that such a person is indeed healthy, and even free of frostbite. Getting checked out can only help. 

FROSTBITE

• Freezing of the Skin - Frostbite sets in during extreme cold when direct exposure to cold effectively freezes your skin and kills the cells. Your extremeties are the most succeptible because of how far away they are from good internal blood circulation, and because of the relatively large surface area exposed to the elements.
• Know the Signs - The signs of frostbite may be obvious, but even the earliest indications are grave causes for warning. The skin will normally start to turn red, but then numbness will also set in. If conditions worsen, complete numbness sets in and the skin will turn pale (or even blue-ish). Under the most extreme conditions, the skin (or even section of the body) will be effectively killed by the cold, perhaps even turn hard and blackish, and require amputation. The difference in time from the first signs to the last are mere minutes in extreme cold, so your first signs are always the best for action, even if out of an overabundance of caution.
• Get Inside and Warm the Area Slowly - Use warm body heat against frostbite, or even a warm compress. Warm the extremety slowly, as a numb hand is still able to be burned in hot water, even if you can't feel it. In this situaiton, warm water is also useful (if applied to an extremety in the safety of a warm area). This type of incubatory warming has the chance to save a part of the body that might otherwise need to be removed.
• Don't Rub Frostbite - Compresses are okay, but if you rub frost-bitten skin, you may cause even more damage. Often in a hopeless situation where frostbite has indeed claimed an extremity, it may be best just to restrict the affected area, rather than risk causing more problems through excessive rubbing. Moreover, in cases of numbness, excessive rubbing/movement can cause damage that might otherwise be prevented through normal sensations of physical pain.
• Seek Medical Attention - This is also a no-brainer. If you feel you have experienced frostbite, regardless of how successful you feel you are in staving off frostbite, getting checked out by a doctor is very important. A trained professional needs to make sure you do not have frostbite (a condition that will persist even after later warming), and a condition that can be extra-hazardous/fatal if not treated properly through further care (unfortunately amputation in many cases).

INSIDE FROM THE COLD:

• Keep Your House Insulated - Preventing draughts from outside is essential to maintaining the temperature inside your home, in addition to keeping your heating bill low. Consider applying weatherstripping to doors, investigating the effectiveness of your storm windows, and, in some situations, blocking the underside of doors with towels. Insulation in the attic (which isn't always present), is also very important, because just like the body loses much heat through the head, so too does the attic of your house in response to cold.
• Be Very Careful When Heating A Home - Many fires occur in Wintertime, and a lof of those are a direct result of trying to get too warm impatiently. Ratcheting up the thermostat can cause a fire danger, or a breakdown of your furnace. Also, handling and using matches in these situations is very hazardous, especially if the gas is turned on high in hopes of some quick warming. Also, building a fire in the stove that gets too big, or with an improperly vented flue, can also cause unwanted problems. Furthermore, many in-home heaters have an open heat source that can cause fire when things that are desired to be warm are too close to them.
• Be Wary of Carbon Monoxide - Carbon Monoxide is a silent killer that mainly strikes in the winter as a direct result of equipment designed to heat. Poorly vented heaters, poorly circulated rooms, and chimney flues that need repair can pose life-threatening hazards. Most of the time you won't know when Carbon Monoxide poisoning is occurring. Consider getting a Carbon Monoxide detector(s) for your home. They can sense a harmful increase in the gas brought on by mostly natural factors related to heating. If there's a Carbon Monoxide alarm, be very careful, because you may end up being safer outside the house, or perhaps in an area of the house with the windows open and great circulation involving outdoor air (that forces you to be prepared for the cold you just came in from).