The Hidden Risk of Keeping Plastic Water Bottles in Hot Cars for Your Dog

If you’re a dog guardian in Australia, chances are you’ve done it (or know someone who has): you keep a few plastic water bottles in the car “just in case” — for after walks, beach runs, park play, or road trips.

The intention is caring. But there’s a problem most people don’t realise:

Heat + time + plastic can change what’s in the water.

In Australian conditions, cars can heat up fast — and bottles left in hot cars for days or weeks may expose your dog to harmful chemicals and plastic particles.

This isn’t about fear. It’s about reducing avoidable exposures — especially for animals with smaller bodies and repeated daily intake.

Why hot cars make this worse

Plastic isn’t inert. Under normal conditions, many plastics release very small amounts of compounds. But higher temperatures and longer storage can increase the migration of certain substances from the bottle into the water.

A hot car is basically a mini greenhouse. Even when it doesn’t feel “that hot” outside, the inside of a parked car can reach temperatures that accelerate chemical migration and plastic breakdown.

If you’re storing bottles for days or weeks, you’re not dealing with a single exposure — you’re creating a repeated exposure habit.

What can leach into the water?

Different plastics have different risks, and not every bottle behaves the same way. But research has repeatedly found that heat and storage time increase migration of certain substances from plastic packaging into liquids.

Here are the main concerns relevant to water bottles:

1) Chemical migration (including hormone-disrupting chemicals)

Some plastics or manufacturing residues can release chemicals into the water, and temperature is one of the key factors that increases this migration.

Even “BPA-free” doesn’t mean “chemical-free.” BPA is only one chemical family; plastics can contain other additives and residues.

Research shows higher temperatures can increase BPA levels migrating into water from certain plastic containers.

2) Antimony leaching from PET plastic bottles

Many single-use water bottles are made from PET plastic (recycling code #1). PET manufacturing commonly uses antimony-based catalysts, and studies have shown antimony can migrate into the water, especially as temperature increases.

One open-access study reports that antimony leaching is low at around 25°C, but rises rapidly around 50°C and above — temperatures that can occur in hot storage environments.

(Important context: this doesn’t mean every bottle becomes “toxic,” but it does mean heat can increase measurable migration.)

3) Phthalates and other plastic additives (especially with heat/sunlight and reuse)

Some research indicates sunlight exposure, higher temperature, and longer storage can increase migration of phthalates from PET bottles, particularly when bottles are reused or stored warm.

4) Microplastics and nano-plastics in bottled water

Bottled water can contain micro- and nano-plastics. A 2024 paper reported very high particle counts in bottled water samples and suggested many particles come from packaging and processing. PNAS+1

Scientists are still working out what these particles mean for long-term health, but many experts recommend reducing unnecessary exposure where practical.

Why dogs may be more vulnerable than humans

Dogs are not tiny humans. A few reasons they can be more sensitive:

• Smaller body size → a “small amount” of exposure can be proportionally larger.

• Repeated habits → if your dog drinks car-stored plastic bottle water frequently, small exposures can add up over time.

• Existing health issues → dogs with gut issues, allergies, liver burden, endocrine problems, or senior dogs may be less resilient.

We can’t say “this bottle will cause disease,” but we can say: this is a preventable exposure with easy alternatives.

What about “forever chemicals” (PFAS)?

You may have heard “forever chemicals” mentioned with plastics. PFAS are persistent chemicals found in many environments and products — but direct evidence specifically showing PFAS leaching from single-use plastic water bottles because they were left in a hot car is limited.

Where PFAS does intersect with this topic is in the broader reality that microplastics and persistent chemicals can co-exist in the environment, and some research suggests combined exposures can be more harmful in ecological models.

For a dog-guardian safety message, it’s usually more accurate to focus on what is clearly supported: heat can increase chemical migration and plastic particle shedding, and repeated exposure is worth avoiding.

The other hot-car issue: bacteria (especially if bottles are opened)

Even if a bottle starts sealed and clean, once it’s opened — or if you pour water into a dog bowl and then top it up later — bacteria can grow much faster in warm conditions.

This is a separate risk from chemical leaching, but it’s another reason why “car water bottles for weeks” is not ideal.

Practical, dog-friendly alternatives (easy swaps)

If you want a simple rule:

Don’t store single-use plastic water bottles in the car long-term — especially through summer.

Instead:

• Bring fresh water each outing (even if it’s tap water in a clean bottle).

• Use a stainless-steel bottle (highly durable, great for cars).

• Use glass if you’re careful with breakage.

• Store water out of direct sun and not in a parked hot car.

• Clean reusable bottles regularly (biofilm builds up fast).

Quick checklist for dog guardians

If the bottle has been:

• sitting in the car for days/weeks

• repeatedly heated and cooled

• exposed to sunlight

• reused many times
…it’s time to ditch it and refill fresh.

Bottom line

Keeping water in the car for your dog is thoughtful — but keeping it in single-use plastic bottles for long periods in Australian heat isn’t ideal.

A small habit change (fresh water + stainless steel bottle) is an easy way to reduce:

• unnecessary chemical exposure

• plastic particle exposure

• and warm-temperature bacterial growth

Our dogs deserve clean, cool water — especially after an adventure.

Research summary

1) Antimony migration from PET bottles increases with heat
Al-Omran, A. M., Ibrahim, A. A., Alharbi, A. R., & Alajmi, H. (2021). The effect of temperature and storage time on the migration of antimony from polyethylene terephthalate bottles into water. Environmental Science and Pollution Research. (Open-access via PubMed Central).

2) Temperature can increase BPA leaching into bottled water
Alhassan, A. J., Sule, M. S., Atiku, M. K., Wudil, A. M., Dangambo, M. A., Mashi, J. A., & Ibrahim, A. (2022). Influence of temperature on the quantity of Bisphenol A leached from bottled water. International Journal of Environmental Health Research. (Open-access via PubMed Central).

3) Heat/sunlight and storage conditions influence BPA release from containers (review)
Sadrabad, E. K., et al. (2023). Bisphenol A release from food and beverage containers: a review. (Open-access via PubMed Central).

4) Sunlight/temperature and time increase migration of phthalates from PET bottles
(Study identifying increased phthalate migration into water from PET bottles with sunlight exposure and warmer storage.)

5) Microplastics and nanoplastics detected in bottled water (2024)
Study reported high levels of micro- and nanoplastics in bottled water samples using advanced imaging/spectroscopy methods. (Peer-reviewed; supported by NIH summary and primary publication.)