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In the article Think big, start small, it's claimed that steel bottles are more sustainable than plastic ones:

If you can drink the water, travel with a refillable water bottle, preferably made of stainless steel (it's better for you and the environment than plastic). If you do buy bottled water or other drinks, see if you can give your empty bottles to local people to reuse.

I'm slightly distrusting of this advice, as it may be that the author is biased in favour of steel over plastic because the latter is even more "unnatural" than the former.

I've heard that plastics being dumped in the environment can cause problems, but I wouldn't just throw a bottle away in the middle of a rainforest.

Is a steel bottle inherently more sustainable than a plastic one? Or is this piece of advice assuming that a steel bottle will be re-used more times than a plastic bottle?

If so, what is the ratio of the number of times that a plastic bottle is re-used versus the number of times a steel bottle is re-used for the two to have an equivalent impact on the environment (say in terms of energy footprint)?

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First of all, it sounds like there might be some confusion regarding the claim.

As I read it, the claim is not that steel is inherently better than plastic. It's not an apples-to-apples comparison. The claim is that one reusable steel bottle is better than continually buying many disposable plastic bottles. Certainly, one steel bottle has a much bigger impact than one plastic bottle, but if you buy a nice $10 stainless steel water bottle, hopefully it'll get used over and over again.

As you might suspect, this depends on many factors. You can probably find many analyses online, but all of them will have to make some assumption about the rate at which bottles are recycled. That may not be relevant to your decision. You probably know how likely you are to recycle plastic bottles, which might be much more than the average person. For that average person, giving them a shiny possession (the steel bottle) might help discourage them from throwing away a seemingly cheap plastic bottle.

One reasonable write-up of this issue was one I found on the NY Times site. Their conclusion was:

What it comes down to is this: if your stainless steel bottle takes the place of 50 plastic bottles, the climate is better off, and if it gets used 500 times, it beats plastic in all the environment-impact categories studied in a life cycle assessment.

Can you reuse a steel bottle 50 times (the climate change balance point)? Or do you lose bottles/lids too easily (or do like me and drive off with them on top of your car)? And, again, do you religiously recycle plastic bottles, or only do so if a recycle bin is convenient? Also, I'm going to plug another question I asked here, as I think it's relevant, if you recycle via public bins.

Also, try to remember that in general, with sustainability, you want to try to reduce consumption first, if you can't do that, reuse resources second, and only if that's not practical, recycle last. Recycling is an imperfect process, especially with plastic bottles.

Finally, it's worth noting that although not strictly a sustainability issue, many believe stainless steel bottles to be better for your health, as a result of fewer harmful substances leaching into your water, compared to plastic bottles.

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    What if I reuse (the word I used in the question) the plastic bottle, rather than merely recycling it? – Andrew Grimm May 22 '13 at 6:59
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    My family has several stainless steel bottles. This brand offers replacement lids, which is fantastic. Bottles we bought 2 years ago are still going strong, being used about twice a week. To help the lids last longer, I recommend buying a type that doesn't require/encourage pulling open with your teeth (it damages the plastic over time). – Highly Irregular May 22 '13 at 9:15
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    +1 for pointing out the importance of assumptions that commonly influence the conclusions drawn from such studies – Stockfisch May 22 '13 at 9:25
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    @AndrewGrimm, this is probably more of a health issue than a sustainability one (although I guess maybe your own health is simply a more narrowly-defined sustainability issue!) ... but, check out this link I included in my answer. It addresses some problems with reusing plastic bottles, as opposed to disposing of them (including recycling). You're certainly right, though, that reusing the bottles improves the sustainability of the plastic bottle. – Nate May 22 '13 at 9:39
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    To strengthen the point in your quote from the NY Times: I think for someone concerned about sustainability 500 uses is a pretty low estimate for a steel bottle. And the caps that you have to remove for each drink don't wear out easily, either. 6 years of use and abuse haven't put mine out of commission. – Evan Johnson May 22 '13 at 15:27
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There are a lot of factors and different ways to view environmental impact and it depends on your priorities. If you prioritize energy used to produce, assuming Nate's citation is correct, the steel is only good when the energy required to produce (mine, refine, smelt, forge, transport, etc.) is less than the energy used to produce an equivalent amount of plastic bottles. Keep in mind, plastic water bottles are a cheap commodity but a pallet of water (35 bottles, 10 per layer, 6 layers) contains over 2300lbs of water alone (without plastic). The plastic also has to be recycled or is discarded into a landfil, if not thrown on the side of the road. Energy consumption seems to be the trendy environmentalist action but I'd argue unsustainable material waste is more important.

Stainless steel is infinitely recyclable, will break down in a healthier manner (I mean that it will totally degrade eventually, unlike plastic which will eventually degrade into microplastics. This is somewhat of an unfair comparison (reusable stainless steel bottle vs single-use plastic water bottles). As long as the reusable product's life isn't terminated shortly, it will be the winner.

Now say, a reliable and reusable plastic bottle (say a Camelbak Eddy) will likely break faster vs a Hydro Flask. I've had my Eddy for 6-7 years, same bottle, same lid, same straw, with only the bite-valve replaced once. I have two Hydro Flasks (one for 5ish years and the other for 4ish years. I use them at least 5 days a week, netting up to 72 fl oz of water, equivalent to 4.2 standard water bottles (16.9 fl oz). If I used 4.2 water bottles a day for 5 days/week (260 days) over 4 years, that uses almost 4200 plastic water bottles, which you must also account for the lids which most facilities don't accept, the bottles that are rejected due to contaminated batches, and services that simply throw recyclables into landfills.

I would always opt for the stainless steel as long as you don't plan on tossing the bottle after a miniscule number of uses. Considering plastic takes hundreds of years (with varying lengths per source, from 100 to 500 years), either way, the plastic remnants will stay around for decades and may pose an environmental hazard to lifeforms that ingest it. If a plastic bottle is used for 50 years, it will still have a great environmental footprint if not recycled properly and completely.

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