Is it safe to drink bottled water left in a hot car?

It’s a common scenario – you leave a bottle of water in your car on a hot, sunny day. When you return, you’re thirsty and tempted to drink that water. But should you? Is it still safe to drink, or has the heat and sunlight caused any harmful changes to the water?

Quick answers

  • Bottled water left in a hot car for an extended period can become unsafe to drink due to chemical changes from the plastic bottle and bacterial growth.
  • The hotter the interior of the car, the greater the risks. Temperatures inside a closed car on a sunny day can exceed 150°F.
  • Clear plastic water bottles are more vulnerable to chemical leaching compared to colored plastic when heated.
  • Water itself does not “expire” but over time, chemical changes can occur that create a bad taste or odor.
  • If the water tastes or smells abnormal, it is best not to drink it. Trust your senses.

We’ve all been there – it’s a hot summer day, you’re thirsty after running errands, and you spot an unopened bottled water in your car. While it might be tempting to crack it open and guzzle it down, is it really safe to drink? Let’s take a closer look at how heat can affect bottled water and when it’s best to toss it out.

How hot do cars get on sunny days?

To understand how heat can impact bottled water, it helps to know just how hot the interior of a parked car can get. On a sunny summer day, even with the windows cracked, the inside of a car essentially becomes an oven.

Some key facts on hot cars:

  • Interior temperatures can climb by an average of 40°F within an hour, even if the exterior temperature is only 70°F.
  • On an 80°F day, the interior temperature can soar to 99°F within 10 minutes.
  • In just one hour, the interior temperature can reach as high as 123°F.
  • Closed cars left in the sun have been recorded as high as 172°F.

The greenhouse effect causes this sharp temperature rise inside a closed vehicle. Visible sunlight easily penetrates the car windows and heats up the interior surfaces. But that accumulated heat has a hard time radiating back out through the glass windows.

For bottled water, those extreme temperatures mean bad news. The higher the heat inside the car, the greater the risks to water quality and safety.

Does heat cause chemical changes in bottled water?

Yes, heat can cause chemical changes to occur in both the plastic of the water bottle and the water itself. Here’s what you need to know:

Plastic bottle breakdown

The most concerning change is chemical leaching from the plastic bottle into the water. Plastics are made of long chains of molecules called polymers. When heated, those polymer chains can start to break down.

Small molecules and additives in the plastic can be released into the bottled water if the plastic breaks down. This is known as leaching.

Studies have found increased chemical leaching from PET plastic water bottles exposed to heat. PET (polyethylene terephthalate) is the most common type of plastic used for commercially bottled water.

Water degradation

The water itself can also undergo chemical changes when heated. Here are two examples:

  • Loss of chlorine: If your bottled water contains disinfecting chlorine, this can dissipate faster when the water is heated. The chlorine helps prevent bacterial overgrowth.
  • Increased metal leaching: Heat can accelerate trace amounts of metals like antimony leaching from the water bottle material into the water.

These chemical changes can lead to altered taste, smell, and safety of the water when left in a hot car.

Does heat promote bacterial growth in bottled water?

Along with chemical changes, heat can also accelerate bacterial growth in bottled water over time. Some key points:

  • Bacteria need water, nutrients, and favorable temperatures to thrive. Bottled water provides the first two.
  • Heat speeds up bacterial division and replication. Many bacteria prefer warmer conditions.
  • Without protective chlorine, bacteria in the bottle can proliferate to unsafe levels after prolonged heat exposure.
  • Common bottled water bacteria include pseudomonas, E. coli, and coliforms.

One study examining water bottles left in cars in Arizona for 2 weeks found bacteria levels increased as interior temperatures went up. Significant levels were detected at averages of 100°F and higher.

When in doubt, it’s best to avoid drinking bottled water that has baked in a hot car and may contain elevated bacteria levels.

Which bottled waters are most vulnerable to heat damage?

Not all bottled waters are equal when it comes to heat and hot car conditions. Here are some vulnerability factors:

Plastic color

Clear plastic water bottles are more vulnerable to leaching compared to colored plastic when heated:

  • Darker plastics absorb more heat energy, protecting the water.
  • Clear plastic allows more light through, heating the water more.
  • Colored bottles like green or blue also help prevent light exposure.

Plastic thickness

Thinner plastic bottles generally have higher risks of chemical leaching and bacterial growth:

  • Thick plastic provides a better heat barrier.
  • Thin bottles have less plastic between the water and high temperatures.

Plastic type

The common PET plastic has more potential to leach chemicals when heated than some other types of plastic such as HDPE (high-density polyethylene).

Sun exposure

Direct sun exposure leads to the highest interior temperatures. Bottled water left on seats or dashboards are most vulnerable:

  • Shaded water bottles are heated significantly less.
  • Tinted windows also help block solar radiation.

In general, thinner, clear plastic water bottles left in direct sun are at greatest risk of heat-related problems.

Does bottled water expire or go bad?

Unlike milk or veggies that naturally spoil, bottled water itself does not technically expire. Water molecules are stable – they don’t break down over time.

However, bottled water can become unsafe to drink if left for too long. Heat exposure in a hot car accelerates this process. Here are some signs your old bottled water may not be good to drink:

  • Smell – If the water has a musty or abnormal odor, it’s best not to drink it.
  • Taste – If the water tastes off (metallic, plastic, etc.), it could be chemically contaminated.
  • Color – Cloudy or discolored water can mean bacterial overgrowth or chemical leaching.
  • Algae – Greenish tint and visible globs or strands mean algae growth.

Trust your nose and taste buds. If the water seems “off” in any way after prolonged heat exposure, play it safe and toss it out.

How long can bottled water safely stay in a hot car?

There is no set time limit for how long bottled water will remain safe in a hot vehicle. It depends on multiple factors, including:

  • Interior temperature of the car
  • Extent of sun exposure/shading
  • Color of the bottle
  • Thickness of the plastic
  • Initial quality and purity of the water
  • If the bottle has been opened previously

As a very general guideline:

  • Unopened water can be risky if left for over a week in hot conditions.
  • An opened bottle should not be left more than a day before tasting questionable.
  • A day or less is safest, if the interior temperature was very high.

But the lower the car temperature and shorter the time period, the less risk there is of heat-induced chemical or bacterial hazards in bottled water.

Tips for storing bottled water in a car

To help minimize heat-related risks for bottled water left in vehicles:

  • Park in the shade whenever possible, or use sun shades.
  • Keep water on the floor or in cup holders away from direct sun exposure.
  • Choose green, blue, or darker colored bottles over clear.
  • Look for thick rather than thin bottle plastic.
  • Try insulating the bottles, such as in a cooler.
  • Never leave open bottles in the car for more than a day.
  • Replace water bottles frequently, don’t let them sit for weeks on end.

Taking simple preventative steps can help minimize the chances of chemical leaching or bacteria growth when water must be left in a car.

Should you ever drink bottled water left in a hot car?

In an emergency situation where no other clean water source is available, drinking questionable bottled water may be necessary for survival. However, in general daily life, discretion is advised.

If the bottle was left for more than a day or two in extreme heat, it’s best to be cautious and avoid drinking it. The plastic and water degradation risks outweigh the benefits, even when you’re really thirsty.

If choosing to drink the heated water in a pinch, some tips include:

  • Smell and taste test it first.
  • Drink only a small amount at first to check for adverse effects.
  • Never drink if the water is discolored, smelly, or tastes off.

Whenever possible, it’s smarter to play it safe and avoid drinking bottled water that’s been baking for long unknown periods in a hot vehicle. The potential risks to your health just aren’t worth it.


Does heat really make bottled water dangerous? In summary, the hotter the car and longer the water has been sitting, the higher the risks of chemical or bacterial contamination.

While bottled water won’t necessarily “expire” on its own, heat exposure can accelerate plastic degradation leading to chemical leaching, dissipate protective disinfectants like chlorine, and allow rapid bacterial growth over time.

Check the bottle for smells, tastes, or discoloration. When in doubt, don’t drink bottled water left for more than a day or two in high interior temperatures. The health risks outweigh the benefits of drinking questionable water.

Prevention is key. Try parking in the shade, using reflective sun shields, choosing opaque bottles, replacing water frequently, and of course – remembering to take your bottled water with you!

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