Mercury thermometers are largely discontinued due to the severe health and environmental risks posed by mercury, a neurotoxin, with many countries and US states banning their sale and use, pushing industries and consumers towards safer alternatives like digital or alcohol thermometers, a transition supported by agencies like the EPA and NIST.

Why They Were Discontinued

Health Hazards: Mercury is a potent neurotoxin; accidental breakage releases toxic vapor, especially dangerous for developing brains in fetuses and children, affecting cognition and learning.

Environmental Contamination: Spilled mercury from broken thermometers is a significant source of environmental mercury, contaminating water and fish.

Regulatory Bans: Many US states (e.g., Illinois) and other countries have banned the sale and distribution of mercury fever thermometers to protect public health and the environment.

Loss of Calibration Support: The National Institute of Standards and Technology (NIST) stopped calibrating mercury thermometers, making it harder for labs to ensure accuracy and forcing a switch.

Current Status: While still found in some older homes or specialized industrial settings, mercury thermometers are generally phased out in retail and healthcare, with efforts focusing on removal and recycling of old devices.