Heating water in a microwave oven can be a real timesaver when it comes to making a warm beverage such as tea. The only real safety concern comes down to ensuring the water is not too hot to drink, as it can reach extreme temperatures when heated in a microwave.
Superheated Water
Superheated water may sound like something designed for a science project or a superhero-in-the-making, but it can actually be quite dangerous. In a microwave, water can reach temperatures beyond the usual boiling point without actually coming to a boil. This "superheated" water is extremely dangerous, since an action as simple as moving the cup or adding a powdered drink mix to it could cause the water to erupt out of its container. Extremely hot water can cause burns.
To prevent overheating the water, heat it only for the amount of time recommended in the microwave owner's manual. The amount of time may vary based on the wattage of the oven and the amount of water being heated. In a typical 500-watt microwave, for instance, 60 seconds heats the water to a warm enough temperature to make tea. Hold the heated cup or mug and take a sip of water if the cup feels warm but not too hot to handle. If it's too hot to hold comfortably, let it cool for several minutes before attempting another sip.
Radiation Concerns
Microwave ovens heat foods and liquids by using a process called non-ionizing radiation. Non-ionizing radiation vibrates the water or liquid molecules enough to cause heat that warms both liquids and the moisture within many foods. This type of radiation doesn't change the chemical structure of liquids or foods, nor does it make liquid or food radioactive in any way.
For Clean Water Only
Microwaving water to drink is only a good idea if the water is already drinkable. Clean tap water, bottled water and filtered water are all drinkable, whether microwaved or not. Microwaving stream water, melted snow or water collected in a rain barrel will not necessarily remove germs, pathogens or chemicals from the water. If snow, for instance, is taken from a pristine location far from pollution and other contaminants, it may be safe to drink but isn't recommended without purification.
Sometimes, government officials issue local boil alerts after working on nearby water pipes, encouraging those in the affected area to boil tap water before using it for cooking or drinking purposes. Boiling it by stovetop is preferable to boiling in a microwave, as the bubbling is a sure sign that the water boils. If a microwave is the only option, place a glass rod or wooden spoon in the vessel along with the water when heating it to prevent the dangers of superheated water. Allow the vessel to cool completely before handling.