Months of low sun, deep cold and summer smoke from big wildfires.
Alaska sits far enough north that winter sun is too weak to make vitamin D in your skin for months. Extreme cold is a daily hazard much of the year, and summer wildfires can fill the air with smoke for weeks.
UVB from the sun is what lets your skin make vitamin D, and it fades with latitude and season. A classic study found no vitamin D made from winter sun at 52 degrees north from October through March. Almost all of Alaska lies farther north than that, so the stretch with little or no vitamin D from sunlight is even longer. Food sources and a conversation with your doctor matter here.
Long stretches well below freezing put you at risk of frostbite and hypothermia, especially with wind, wet clothing or a vehicle breakdown far from help.
Summer wildfires in Alaska and western Canada can send smoke far from the flames, sometimes for weeks. Smoke is full of fine particles that reach deep into your lungs, causing coughing, burning eyes and headaches, and it can trigger asthma and heart problems.
UV reflects off snow, so you can burn your skin and eyes in late winter and spring even when it is cold.
Numb, white or waxy skin after cold exposure, or shivering that stops while you are still cold, needs medical care right away. See a doctor for trouble breathing or chest pain during smoky days.