Much has been stated in the past few decades about the dangers of mercury and the chemical's use in thermometers. It is true that mercury can be a dangerous chemical, though there is some dispute about how dangerous it is if properly cleaned up. As far as affordable thermometers go, the mercury thermometer is the most accurate and has several other advantages over alcohol-based thermometers.
When it comes to thermometers, like most things, you get what you pay for. Alcohol (also called spirit-filled) thermometers are inexpensive but they also break very easily. The thread of the thermometer is quite fragile, and when it breaks it is nearly impossible to rejoin. Mercury thermometers tend to be much more durable under duress.
Because of sensitivity of temperature, high thermal conductivity, low thermal capacity, good visibility and non-wetting of the internal capillary walls of the thermometer, mercury is typically a very accurate gauge of temperature. Alcohol thermometers, on the other hand, tend to have a lower conductivity while wetting the walls of the device. This can make the thermometer show the results more slowly and less accurately. Some believe that non-mercury thermometers should not be used in secondary schools because of this inaccuracy.
Mercury has been used in thermometers for many years due to its wide temperature range. Mercury functions in the range of 234 Kelvin to 630 Kelvin -39 to 356 degrees Celcius, -38 to 674 degrees Fahrenheit), which cannot be matched by alcohol thermometers. Despite this wide range, the amount that mercury expands is eight times less than alcohol, which allows mercury thermometers to be made smaller and more compact than alcohol thermometers. Mercury's high boiling point allows it to be used in thermometers that are in steaming hot liquids and other cooking foods.
The disadvantages of mercury are quite clear. While inhaling vapours of the metal is most dangerous with constant exposure, getting any mercury on your skin can be very harmful. Mercury is difficult to clean and it can easily contaminate other surfaces after a thermometer breaks. Mercury has a low freezing point relative to alcohol (- 3.89 degrees C centigrade, 25 degrees F) and as a result it cannot be used in colder areas. If an extremely accurate reading is not necessary, purchasing an alcohol-based thermometer will be cheaper and less dangerous.