Warm tap water is generally not recommended for consumption. Residential plumbing systems are designed to deliver cold water for drinking and cooking purposes. Using the hot water faucet for any consumable application introduces significant risks not present when using cold water. These primary concerns involve the accelerated leaching of heavy metals from plumbing materials and increased microbial growth within the home’s water storage system.
Accelerated Leaching of Pipe Contaminants
Warm tap water is discouraged for drinking due to the chemical interaction between heated water and the pipes it travels through. Higher water temperatures dramatically increase the rate of corrosion, causing plumbing materials to dissolve into the water supply. This heat-accelerated process causes heavy metals to leach more readily into the water than they would under cold conditions.
Metals of concern include lead and copper, which can be present in pipes, solder, and brass fixtures. Studies indicate that increasing the water temperature from 60°F to 140°F can accelerate the corrosion rate of steel significantly. Hot water is more likely to contain elevated levels of these substances, which are harmful if ingested.
Leaching occurs because heat disrupts the protective mineral scale that naturally forms on the inside of pipes, exposing the metal underneath. Even “lead-free” plumbing components may contain trace amounts that are more easily dissolved by warm water. Furthermore, the leaching of various chemicals from plastic pipes and fittings is also intensified when the material is exposed to hot water.
Bacterial Risks from Water Storage
The second major health risk involves biological contamination, specifically originating from the water heater or hot water tank. Unlike cold water, which is constantly refreshed from the municipal line, hot water sits in a tank where conditions can become ideal for microbial growth. Stagnant water kept at a warm, but not fully hot, temperature poses a particular risk.
The ideal temperature range for the growth of Legionella bacteria, which causes Legionnaires’ disease, is between approximately 77°F and 113°F (25°C to 45°C). Water heaters often contain portions of water, especially near the bottom of the tank, that remain within this warm range, allowing the bacteria to thrive.
Drinking contaminated water can pose a risk to vulnerable individuals, although inhaling aerosolized Legionella during a shower is the most discussed danger. To kill the bacteria effectively, water must be heated and stored above 140°F (60°C). Prolonged storage at suboptimal temperatures within the tank creates a biological hazard not present in the fresh, cold water supply.
Safe Alternatives for Hot Water Use
The safest practice for all consumption purposes is to draw water exclusively from the cold water tap. This ensures the water has not passed through the hot water heater or sat in the pipes long enough to accumulate high levels of leached contaminants. This rule applies universally to drinking, cooking, washing produce, and preparing baby formula.
If hot water is needed for consumable use, the proper procedure is to first draw cold water and then heat it separately using a stove or an electric kettle. Boiling water is effective for killing bacteria, but it will not remove heavy metals like lead. Boiling water can potentially concentrate lead levels due to the evaporation of the water itself.
Before drawing cold water for consumption, especially after a long period of disuse, it is beneficial to flush the line. Running the cold water tap until the temperature is noticeably colder clears out any water that has been sitting in the immediate plumbing and may have picked up trace amounts of metals. Warm tap water remains acceptable for non-consumable uses, such as washing dishes, showering, or cleaning.