Although thermometers are generally safe, improper use, poor hygiene, damaged devices, or outdated thermometer types can occasionally lead to unwanted side effects or health risks. Fortunately, these risks are easy to prevent with proper handling and cleaning.
This comprehensive guide explains everything you need to know about thermometer side effects, different thermometer types, safe usage techniques, hygiene practices, and expert recommendations for accurate temperature measurement.
What Is a Thermometer?
A thermometer is a medical instrument used to measure body temperature. It helps detect fever, hypothermia, and monitor various medical conditions.
Modern thermometers include:
- Digital thermometers
- Infrared forehead thermometers
- Ear (tympanic) thermometers
- Oral thermometers
- Rectal thermometers
- Basal body thermometers
- Smart Bluetooth thermometers
Older mercury thermometers are no longer recommended in many countries because of mercury toxicity if broken.
Are Thermometers Safe?
Yes.
When used correctly, thermometers are extremely safe and rarely cause any serious complications.
Most reported problems occur because of:
- Improper cleaning
- Sharing between people
- Incorrect insertion
- Damaged devices
- Mercury exposure
- Reading errors
Can Thermometers Cause Side Effects?
Yes—but serious side effects are uncommon.
Potential issues depend on the thermometer type and how it is used.
Common side effects include:
- Skin irritation
- Ear discomfort
- Oral irritation
- Minor rectal injury
- Infection transmission
- Anxiety from inaccurate readings
- Mercury poisoning (broken mercury thermometers)
Types of Thermometers and Their Possible Side Effects
1. Digital Thermometers
Digital thermometers are the safest and most commonly recommended option.
Possible Side Effects
- Mild discomfort
- Cross-contamination if not cleaned
- Incorrect readings if positioned improperly
Overall risk: Very low
2. Infrared Forehead Thermometers
These measure temperature without touching or with minimal contact.
Possible Side Effects
- No radiation risk
- No tissue damage
- Reading errors from sweat or sunlight
Many people mistakenly believe infrared thermometers expose users to harmful radiation.
This is false.
Infrared thermometers only detect naturally emitted body heat—they do not emit harmful radiation into the body.
3. Ear Thermometers
Ear thermometers measure heat from the eardrum.
Possible side effects include:
- Temporary ear discomfort
- Incorrect readings if inserted incorrectly
- Minor irritation in sensitive ears
Not recommended for infants younger than the manufacturer's specified age.
4. Oral Thermometers
Oral thermometers are accurate when used properly.
Possible side effects:
- Temporary gagging
- Minor mouth discomfort
- Spread of germs if shared without cleaning
Avoid measuring immediately after:
- Hot drinks
- Cold drinks
- Smoking
- Chewing gum
Wait 15–30 minutes for accurate results.
5. Rectal Thermometers
Rectal temperature measurement is considered one of the most accurate methods, especially for infants.
Possible side effects:
- Mild discomfort
- Small risk of rectal injury if inserted improperly
- Cross-contamination
Always use lubrication and never force insertion.
6. Mercury Thermometers
These are rarely recommended today.
Possible Side Effects
- Mercury exposure
- Toxic vapor inhalation
- Environmental contamination
- Glass breakage injuries
If a mercury thermometer breaks:
- Leave the room if possible.
- Avoid touching mercury with bare hands.
- Never use a vacuum cleaner to clean mercury.
- Follow your local public health guidance for cleanup and disposal.
Infection Risks from Thermometers
One of the biggest concerns is cross-contamination.
Bacteria and viruses may survive briefly on thermometer surfaces if they are not cleaned properly.
Sharing thermometers without disinfection may increase the chance of transmitting germs.
Risk is higher when using:
- Oral thermometers
- Rectal thermometers
- Ear thermometers
Lower with:
- No-touch infrared thermometers
Skin Irritation
Some people may experience mild irritation due to:
- Frequent rubbing
- Alcohol sensitivity
- Latex covers (rare)
Symptoms include:
- Redness
- Mild itching
- Dry skin
Usually these symptoms resolve quickly.
Can Infrared Thermometers Damage the Brain?
No.
This myth became widespread during the COVID-19 pandemic.
Scientific evidence shows forehead infrared thermometers simply detect heat emitted from the skin.
They do not:
- Emit harmful radiation
- Affect brain cells
- Damage DNA
- Alter body temperature
Mercury Exposure: A Serious Exception
Mercury is toxic if inhaled as vapor.
Symptoms of significant exposure may include:
- Headache
- Tremors
- Memory problems
- Mood changes
- Breathing difficulty (high exposure)
This is why digital thermometers have largely replaced mercury devices.
Proper Thermometer Hygiene
Good hygiene greatly reduces infection risk.
Before Use
- Wash hands.
- Inspect the thermometer for damage.
- Use a clean probe cover if recommended.
After Use
- Clean with soap and water if appropriate for the device.
- Disinfect with 70% isopropyl alcohol if recommended by the manufacturer.
- Dry completely before storing.
How to Use a Thermometer Correctly
Digital Thermometer
- Wash hands.
- Clean the thermometer.
- Turn it on.
- Place it correctly (oral, underarm, or rectal depending on the model and intended use).
- Wait for the beep.
- Read the display.
- Clean again before storage.
Common Mistakes That Affect Accuracy
Avoid these common errors:
- Measuring after exercise
- Taking temperature immediately after hot drinks
- Using a damaged thermometer
- Failing to clean the probe
- Incorrect ear thermometer placement
- Using low batteries
- Measuring in direct sunlight with infrared thermometers
- Not reading the instruction manual
Who Should Be Extra Careful?
Extra attention should be taken with:
- Infants
- Older adults
- Immunocompromised individuals
- Hospital patients
- People with ear infections
- Individuals with rectal disorders (when using rectal thermometers)
Benefits of Proper Thermometer Use
Proper use provides:
- Accurate fever detection
- Early illness recognition
- Better treatment decisions
- Reduced infection spread
- Safe monitoring at home
- Peace of mind for caregivers
Safety Tips
✔ Wash hands before and after use.
✔ Clean the thermometer after every use.
✔ Never share without disinfecting.
✔ Store in a protective case.
✔ Replace cracked or damaged thermometers.
✔ Keep away from children when not in use.
✔ Use the correct thermometer for the intended measurement site.
✔ Replace batteries when readings become unreliable.
✔ Dispose of mercury thermometers safely according to local regulations.
Frequently Asked Questions (FAQs)
Can thermometers spread germs?
Yes. Shared thermometers can spread bacteria or viruses if they are not cleaned and disinfected between users.
Are infrared thermometers harmful?
No. Infrared thermometers detect heat emitted from the body and do not expose users to harmful radiation.
Can a digital thermometer cause infection?
Not if it is cleaned properly after each use.
Is a mercury thermometer dangerous?
The intact device poses little risk, but a broken mercury thermometer can release toxic mercury vapor and requires careful cleanup.
How often should I clean my thermometer?
Before and after every use, following the manufacturer's cleaning instructions.
Which thermometer is the safest?
Digital thermometers and properly used no-touch infrared thermometers are generally considered the safest options for home use.
Final Thoughts
Thermometers are essential tools for monitoring health and detecting fever. For most people, they are very safe when used as directed. The main concerns—such as infection transmission, inaccurate readings, minor discomfort, and, in the case of older devices, mercury exposure—can largely be avoided through proper hygiene, correct technique, and routine maintenance.
Choosing a quality thermometer, cleaning it after every use, following the manufacturer's instructions, and replacing damaged devices will help ensure accurate readings while minimizing risks. With these simple practices, thermometers remain one of the safest and most reliable tools for everyday healthcare.