Thermometers: How to Tell if You Have a Fever

There are three main types of thermometers, namely: glass thermometers (mercury or ethanol thermometers), digital thermometers, and infrared thermometers.

* It is not recommended to use a mercury-in-glass thermometer anymore as mercury is harmful to humans. It could poison you if the thermometer breaks and the mercury is released. It is hazardous to the environment too, so pay extra attention to its disposal. Please refer to the Environmental Protection Department’s guide on recycling mercury thermometers.

Checking body temperature 

A thermometer is a simple, non-invasive and fast method to monitor body temperature. Elevated or low body temperatures may indicate certain health conditions.

A high body temperature (usually over 38°C or 100.4°F) can indicate:

• Fever: Can be caused by an infection (e.g. COVID-19), medications, certain cancers or severe injuries/traumas, including heart attack or stroke
• Heat exhaustion
• Serious sunburn
• Medical conditions that cause inflammation, such as rheumatoid arthritis
• Metabolic disorders such as hyperthyroidism

A low body temperature (i.e. hypothermia) (usually under 35°C or 95°F) can indicate:

• Overexposure to cold temperatures
• Shock (suddenly passed out, dizzy or unconscious)
• Overindulgence in drugs/alcohol
• Metabolic disorders such as diabetes or hypothyroidism 

Tracking ovulation

Thermometers can also be used to track ovulation, helping women who want to conceive to increase their success rates.

The basal body temperature (BBT) is a person's at-rest temperature. You can measure your BBT in the morning before you get out of bed. Because your temperature varies based on the time of day, your physical activity, and the food and beverages you’ve consumed,  it is important to measure your body temperature by the same method, at the same time, and under the same conditions each day to obtain a comparable result.

Just before your ovary releases an egg, your body temperature decreases a little. Your temperature then increases 24 hours after the egg is released and remains elevated for several days. A woman's BBT is typically 36.1-36.4°C (97-97.5°F) prior to ovulation. After ovulation, it rises to 36.4-37°C (97.6-98.6°F). Women can track their BBT to find out when they are ovulating. 

However, as the body temperature is affected by many other factors, e.g. illness, stress, interrupted sleep cycles, etc, and a woman’s cycle can change, tracking your ovulation alone is not a reliable contraception method.

The directions for using a thermometer depend on its type and the measurement sites, therefore it is best to follow the instructions on the package. Some general directions are listed below.

Using a digital oral thermometer

1. Rinse the thermometer in cold water, clean it with alcohol and rinse again before and after using.
2. Place the thermometer tip under the tongue and close your mouth while measuring.
3. Hold the thermometer in the same spot for about 40 seconds or when the final reading is done (usually indicated by a beep sound).

Using a tympanic (ear) thermometer

1. Rinse the thermometer in cold water, clean it with alcohol and rinse again before and after using. Alternatively, use a protective cover and discard it after each measurement.
2. Gently pull your ear up and back before putting the thermometer in your ear to open the ear canal and obtain a more accurate result.
3. Do not remove the thermometer until the measurement is complete (usually indicated by a beep sound).

Normal body temperature

The "normal" body temperature varies from person to person. The normal body temperature for an adult is between 36.1°C (97°F) and 37.2°C (99°F). The range for infants and young children is 36.6°C (97.9°F) to 38°C (100.4°F), which is a little higher.

Body temperature indicating fever

Since surface temperature is more susceptible to changes in the surroundings, it is considered less accurate for gauging body temperature.

Source: Medical Device Division, Department of Health, HKSAR 
 

In Hong Kong, a thermometer can be purchased from medical device shops, pharmacies, electronic appliances stores and other retail outlets.