Leaving a can of paint locked inside a hot car, especially during the summer, may seem harmless enough. However, subjecting paint to high temperatures can be incredibly dangerous. From explosions and fires to skin burns and inhalation hazards, improperly stored paint can put you and your loved ones at serious risk.
Heat’s Effects on Paint
Before diving into the specific dangers, it helps to understand how heat affects paint. In general, heat causes the ingredients in paint to expand and break down. Different types of paint react differently, but all paints suffer when exposed to extreme heat over time.
Effects on Latex and Water-Based Paint
Latex and water-based paints fare better than oil-based alternatives but still deteriorate. As latex paint heats up, the water begins to evaporate, causing the paint to thicken and become pasty. This makes it very difficult to apply the paint smoothly and evenly. The textures and finishes will likely be uneven and unattractive.
Additionally, some of the latex binders and additives may start separating from the pigment and water. This can make the paint appear curdled. Vigorous stirring may temporarily fix the consistency, but once the paint cools again, the ingredients tend to separate once more. So you end up with a lumpy, impossible-to-use mess.
Effects on Oil-Based Paint
The biggest hazard with oil-based paints is the highly flammable solvents. As temperature increases inside a sealed paint can, the liquid solvents expand into vapor form. This builds pressure inside the can, which can potentially lead to explosion. More on this shortly.
Aside from the explosion risk, the paint quality also suffers. Much like latex paint, the pigments and binders tend to separate and the paint may thicken. Long term heat exposure eventually turns oil-based paint into a lumpy, useless sludge.
Effects on Aerosol Spray Paint
Aerosol spray paint cans face the same pressure build up and risk of explosion as regular cans, but to a much higher degree. The fuels used to aerate spray paint are extremely flammable gasses like propane and butane. Just a few hours in a hot car can cause the pressure inside a spray paint can to spike dangerously. We’ll cover the explosion hazards next.
Risk of Explosion and Fire
Now that you understand how heat affects paint, let’s discuss the biggest danger – explosions and fire. Believe it or not, a can of paint left in a hot car can turn into a dangerous projectile.
Pressure Buildup in Closed Cans
Both oil-based and water-based paints contain solvents that keep the ingredients in liquid form. These solvents turn to vapor when heated, causing pressure to build up inside the can rapidly. The boiling point for most solvents is between 100°F – 200°F. So on a hot summer day, the interior of a car can easily exceed this threshold.
As pressure rises exponentially, it will eventually exceed the strength of the can. The result is an explosive release of pressurized vapors and paint. This can turn the can into a dangerous projectile, spraying paint everywhere. People have been injured and property damaged by exploding cans of paint left in cars or direct sunlight. It’s a very dangerous situation.
Heightened Risk With Aerosol Spray Paint
Pressurized aerosol cans have an even greater risk of explosion versus standard cans. In addition to the solvents boiling, most spray paint contains highly flammable gasses like propane, butane, and isobutane to act as propellants.
These hydrocarbons turn from liquid to vapor very quickly when heated, rapidly expanding in volume. It takes only 10-15 minutes in a hot car for the internal pressure to swell beyond safe limits. The result is an explosion that can literally blow the can apart, propelling shrapnel in all directions.
There are many documented cases of aerosol cans exploding after being left in cars on hot days. In one case, a can left on a dashboard caused an explosion that blew out the windshield. Spray paint cans should never be left in a hot vehicle under any circumstances.
Potential for Fires
In addition to the explosion hazard, heated solvent vapors and propellants can also ignite if exposed to a spark or open flame. Ruptured cans often spray a trail of ignitable vapor and liquid. If ignition occurs, this can create a fast-spreading fire.
Gasoline, lighters, engine heat, exposed wiring, and other ignition sources may exist in or around the overheated vehicle. If paint vapor comes into contact with any of these, a fire erupting is highly likely. Fires spawned by exploding paint cans have caused serious injuries and burned down garages and vehicles.
How Hot Is Too Hot for Paint in a Car?
While any exposure to substantial heat can be problematic, in general:
- Temperatures under 90°F (32°C) are relatively safe for short periods of 2-4 hours.
- Temperatures over 90°F (32°C) can start to degrade paint if left for longer than 2 hours.
- Temperatures over 100°F (38°C) can rapidly damage paint if left for more than an hour.
- Temperatures over 120°F (49°C) are extremely dangerous and can lead to explosion after just 30 minutes.
The interior of a car heats up incredibly fast in summer weather. The greenhouse effect traps heat inside the cabin, with temperatures often soaring over 120-140°F (49-60°C) even when the outside air is only 85-90°F (29-32°C).
Leaving paint cans for any length of time in these kinds of extreme temperatures is hazardous. The hotter it is outside, the faster the inside temperature rises.
How Long Can Paint Safely Be Left in a Hot Car?
As a general guideline:
- In mild 70-80°F (21-27°C) weather, paint might be okay left in a car for 4-6 hours.
- In hot 80-90°F (27-32°C) weather, limit time in the car to 2-4 hours.
- In very hot 90-100°F (32-38°C) weather, paint should not be left for more than 1-2 hours.
- In extremely hot 100°F (38°C) or above weather, do not leave paint in the car for more than 30 minutes.
However, these times are just estimates and depend on many factors:
- Type and formulation of paint product. Oil-based are most vulnerable.
- Color of vehicle interior. Dark colors absorb more heat.
- Direct sun exposure. Partial shade is better than full sun.
- Wind flow. Breezy days don’t heat up as quickly.
- Time of day. Morning is cooler than mid-afternoon.
The safest bet is to not leave paint in a hot car for more than 2 hours, regardless of weather, if you need to keep the paint in good condition. Never leave aerosol cans in hot vehicles unattended.
Health Dangers of Heat-Damaged Paint Fumes
Assuming you avoid an explosion or fire, using heat-damaged paint still poses significant health risks if the fumes are inhaled.
Irritation of Eyes, Skin, and Respiratory System
The solvents used in paint thinner, oil-based paints, spray paints, etc. are largely comprised of volatile organic compounds (VOCs). When vaporized and inhaled, these include chemicals like benzene, formaldehyde, and acetone.
Exposure to high concentrations of VOCs can severely irritate the eyes, skin, and lungs. Some people experience coughing, wheezing, chest pain, and difficulty breathing when exposed to paint fumes. Those with respiratory conditions like asthma are especially vulnerable.
Irritation happens more quickly with heated paint products. The heat exponentially increases the amount of vapor released. Just being around an open can of heated paint is enough to trigger a response for sensitive individuals. Using the paint in a confined, unventilated space makes the effects even worse.
Potential Long Term Effects
Beyond just irritation, inhaling paint fumes daily over an extended period also carries long term health risks. Many of the chemical solvents are known carcinogens and can cause organ damage with prolonged exposure.
Heating paint causes more of these hazardous chemicals to vaporize. Working around damaged paint without proper ventilation and masks puts your lungs, liver, kidneys, brain, and other vital systems at risk.
Children and pregnant women need to be kept far away from any area where heated paint is being used or stored. The developmental effects can be severe.
Recognizing and Disposing of Heat-Damaged Paint
Now let’s move on to some tips for identifying paint that has been compromised by heat, plus safe disposal practices.
Signs of Overheated Latex Paint
Latex paint that has been exposed to excess heat may exhibit the following warning signs:
- Thick, pasty texture that cannot be stirred back to normal
- Grainy, curdled, separation of ingredients
- Foul, chemical odors
- Discoloration or fading
- Hard crust or skin formed on surface of paint
Any of these are indications that the paint is no longer usable and will not apply properly. Do not continue trying to use it.
Signs of Overheated Oil-Based Paint
Watch for these signs of heat damage with oil-based paints:
- Strong chemical odor indicating evaporated solvents
- Thick, greasy texture with poor flow and leveling
- Glossy skin formed on surface of paint
- Separation or coagulation of ingredients
- Darkening color and loss of sheen
As with latex paint, the appearance of any of these characteristics means the paint should be safely discarded. The risks are too high.
Signs of Overheated Spray Paint Cans
With spray paint, focus on the condition of the can itself. Warning signs include:
- Bulging or swelling of can ends
- Dents, cracks, or leaks indicating pressure buildup
- Abnormal hissing or spurting noise when operated
- Gurgling, popping, bubbling noises from agitation
- Wet paint oozing from seams
Any of these point to dangerous levels of pressure inside. Do not use the can. Follow proper disposal protocol covered next.
Safe Paint Disposal
Never throw damaged, overheated paint in the regular trash. The fumes can leak out and ignite other materials. Likewise, don’t try to open the can and pour it down a drain. The chemicals can be hazardous to groundwater and the septic system.
Instead, follow these safe disposal steps:
- Allow the damaged paint to dry out completely with the lid off in a well-ventilated area. This may take weeks.
- Once dried, soak up any remaining residue with cat litter or rags.
- Place the completely dried can and absorbent material in a plastic bag and seal tightly.
- Check for local hazardous waste disposal days and locations to properly dispose of the sealed paint.
- If no hazardous waste disposal is available locally, trash the sealed paint normally.
Never store or transport paint that has been overheated until it is 100% dried out. The risk of fire or poisoning is too severe.
The Bottom Line on Storing Paint in Hot Cars
In summary, here are the key takeaways on the dangers of leaving paint inside hot vehicles:
- All types of paint degrade when exposed to sustained heat over 90°F-100°F
- Oil-based paints and spray paint risk dangerous pressure buildup and potential explosion
- Fumes from overheated paint can cause eye, skin, and respiratory irritation and illness
- Never leave paint sitting in a vehicle unattended for longer than 1-2 hours absolute maximum
- Store paint properly between 60°F-80°F; avoid uncontrolled garages and sheds
- Dispose of heat-damaged paint safely as hazardous waste once dried out
- Read and follow all paint label temperature guidelines for storage and transport
Paint left in cars or direct sun, even just for part of a day, are ticking time bombs. An explosion can cause serious injuries and property damage. Storing paint properly goes a long way towards preventing accidents and health hazards. When working in hot conditions, take the extra time to keep paint cool and out of vehicles. Don’t take unnecessary chances with unstable, volatile paint products. Your safety is too important.
