Leaving a canned soda in a hot car is a common oversight that carries multiple risks. While the beverage may not become immediately unsafe to drink in a life-threatening way, it is strongly discouraged due to physical, chemical, and quality concerns. The heat inside a closed vehicle on a sunny day can quickly exceed 130 degrees Fahrenheit. This environment stresses the can’s structure, degrades its internal lining, and fundamentally alters the liquid inside. Understanding these mechanisms reveals why this mistake should be avoided.
How Extreme Heat Affects Internal Can Pressure
The most immediate and dramatic risk comes from the buildup of pressure within the sealed aluminum container. Carbonated beverages are already under pressure from dissolved carbon dioxide gas, which keeps the liquid fizzy. For a typical soda can stored in a refrigerator at 40 degrees Fahrenheit, the internal pressure is around 17 pounds per square inch (psi) above atmospheric pressure.
As the temperature rises, two physical processes combine to rapidly increase the internal force. First, the carbon dioxide gas that occupies the small headspace expands in volume. Second, the solubility of carbon dioxide in water decreases sharply as the liquid temperature increases, forcing more dissolved gas out of the solution and into the headspace.
A can of cola at 75 degrees Fahrenheit already reaches approximately 55 psi. Inside a parked car, temperatures can soar past 130 degrees Fahrenheit, pushing the internal pressure far higher. Aluminum cans have a structural failure point, typically exceeding 100 psi. When high heat forces enough CO2 out of solution, the combined pressure can overcome the can’s structural integrity, leading to a rupture or sudden explosion.
Chemical Concerns from Liner Degradation
High heat affects the can’s internal protective coating, posing a long-term health concern. Aluminum cans are lined with a thin layer of epoxy resin that acts as a barrier. This resin prevents the acidic nature of the soda from reacting with the metal, which would otherwise cause the beverage to take on a metallic taste and corrode the can.
Many of these epoxy resins historically contained Bisphenol A (BPA), a synthetic compound used to make certain plastics and resins. While many manufacturers have transitioned to alternatives, chemical leaching from the liner remains a concern when exposed to heat. Extreme heat accelerates the breakdown of these resin compounds, prompting the migration of chemical components from the lining into the liquid content.
This migration is especially pronounced in acidic beverages like soda, which can help break down the polymer barrier. Studies have shown that exposing canned goods to high temperatures, such as 212 degrees Fahrenheit, can increase the release of compounds like BPA compared to unheated cans. Consuming a beverage with high concentrations of these leached compounds is undesirable because they can act as endocrine-disrupting chemicals that interfere with the body’s natural hormone systems.
Quality Changes in the Beverage
Extreme heat dramatically reduces the beverage’s quality, making it unpleasant to drink. The loss of the characteristic fizziness is the most noticeable change. Carbon dioxide gas is less soluble in warm liquids, meaning the heat causes the gas to escape the solution and build up in the can’s headspace. When the can is opened, the soda will taste flat because a significant portion of the dissolved carbon dioxide has already been driven out.
For diet or zero-sugar sodas, high temperatures accelerate the thermal degradation of artificial sweeteners, particularly aspartame. Aspartame is chemically unstable when exposed to heat and the low pH of soda. This breakdown results in a loss of sweetness and the formation of byproducts, such as diketopiperazine. The resulting beverage can taste less sweet, or even develop a chemical or bitter aftertaste.