Thread by @arthurhcyip

Did you hear how cotton bags are worse than plastic bags, specifically 7000x worse? Well, the story is much more complicated than that. I find 3 major problems & 2 minor problems, and I fault not only the media but also the LCA study's authors. ... 1/n

First, some bad takes:
-"nobody possibly uses their cotton bags 7000x - cotton is so much worse than plastic"
-"everything we do is bad for the environment (pick your poison!), AND the greenies who love cotton and organic are literally the WORST" 2/n

Some alternative acceptable/good takes:
>Plastic bags aren't that bad in some environmental aspects
>Paper and reusable and recycled plastic are also pretty good bag materials
>Organic can be worse for the environment (due to reduced yield)
3/n

>Reduce, recycle, and re-use remain really relevant & important, & in many cases, much more so than material choice
>Apparently Danes consume, per year, the equivalent of 63 cotton bags in textiles. So one could easily compensate for one cotton bag simply w/ one less t-shirt. 4/n

>Stepping away from plastic bags / cotton bags / straws, etc., have you considered eating less meat and taking fewer car and plane rides? These activities and choices have much more impact on health and environment.
5/n

I agree w/ (/came up with) these takes. But still, the good takes can't seem to resist hyping the numbers, particularly the 7100x and 21000x.
I was surprised by the order of magnitude & looked into the Life Cycle Assessment (LCA) study that was the source of this number.
6/n

Background:
The LCA was commissioned by the Danish Ministry of Environment & written by researchers at @DTUtweet. LCA is a way to compare cradle-to-grave impacts, on an apples-to-apples basis. But environmental impacts are complicated. Some things are really hard to compare! 7/n

Cotton vs. plastic?
Cotton: farming, agricultural & textiles chemicals, fertilizer productn & run-off, transportation, textiles production.
Plastic: oil extraction, refining, plastics manufacturing (but a lot less material is processed bc plastic is weight&volume-efficient). 8/n

LCA studies have typically focused on climate change in the past. This one studied "a comprehensive set of indicators" bc people r rightfully interested in non-climate health and environmental impacts as well as climate. 9/n

Finally, even though math & quantities are involved, which can be more objective than pure opinion/conjecture/feelings, LCA is still inevitably highly subjective bc results are highly subject to assumptions abt everything. 10/n

Particularly, how the results are presented can also paint vastly different pictures and be misleading. Uncertainty and false precision is another problem. 11/n

First, I found Major Problem #1 on pg18, ftnote5.
7100x was the reuse number for ONE impact category, specifically ozone depletion. They reported the maximum numbers in the table,&then misleadingly labelled it "all env impacts" instead of "impact with maximum reuse number"! 12/n

The required reuse number actually varies from 52x - 7100x! 52x is very doable! This is a ridiculously different result than 7100x.
Use a cotton bag once/week for a year & it will have "paid off". We're talking abt nice cotton cloth bags, not the thicker plastic ones. 13/n

Major Problem #2: So a cotton bag needs to be used 7100x to have same impact on O3 depletion as LDPE baseline.
But 7100x of a miniscule number can still be miniscule. They cld both be really harmless! I suspect LDPE (plastic bag) has a tiny O3 impact, & so % diff seems large 14/n

Just because something is 7100x worse, doesn't mean it's that bad either! The absolute value matters too here, not just relative values.
Also, I am certain that uncertainty dominates the estimates of life cycle O3 impact data that resulted in this point estimate of 7100x. 15/n

Now let's check Appendix C.
Cotton vs. LDPE: 1000x for freshwater & terrestial eutrophication. 67-400x for PM, ionising radiation, terrestial acidification, marine eutrophication, freshwater ecotoxicity, cancer, non-cancer toxicity, photochemical ozone formation. 16/n

OK, so health and environmental impacts vary. Again, the 67-400x are much closer to reuse numbers that many cotton bags can meet! 17/n

1358x for water use, 64x for fossil resource depletion, 98x for resource depletion.
Minor Problem #1: I've always found these particular impact categories controversial. Water & resource consumption can be added up, but impacts can vary widely depending on where and how. 18/n

Notably, 52x on climate. After using a cotton bag 52x, cotton has lower climate impact than plastic!
Again, this is very different from "7000x" or even the average number, 840x, which equally weights all the impact categories discussed. 19/n

Minor Problem #2: for organic cotton, study assumed 2 were needed to meet functional unit requirement that 1 LDPE or cotton bag cld meet. They admit "large overcapacity" for some bag types. So organic is not really 3x worse than conventnl (21000/7000), more like 1.6x worse. 20/n

Major Problem #3: They did NOT acct for uncertain health&env impacts of plastic waste/microplastics in ocean/ecosystem. 100% disposal assumed. They likely couldn't assess this w any desired accuracy or precision. But this is the issue that ppl r most worried abt re plastic! 21/n

If 99% plastic bags reach proper disposal, one might add 1% of their calculated impact to the numbers they produced, BUT this assumes they somehow included littered plastic impacts. This would include things like possible cancer risk from ingesting some fish that had 22/n

ingested microplastic. But there r just no accurate or precise estimates of these impacts. So this is a major limitation. Study&most coverage admit this. But unaccounted & out of scope doesn't mean zero impact, which the "7000x" implies, given their analysis specification. 23/n

I find that some of the problems I identified were media's and social media's fault. There is a lot of pressure, profit, and enthusiasm for clickbait and hippie-punching. 24/n

But I also fault the authors of this study. Yes, they did identify and address a few of the assumptions I mentioned. There was sensitivity analysis. This study improved on previous studies by leaving "reuse number" as a parameter rather than an assumption. 25/n

But they left too much unsaid than was appropriate. They did not emphasize the severe limitations or address alternative explanations & interpretations that I discussed. Authors state that study did not pass ISO14040 for 1 reason - it was not reviewed by panel of experts. 26/n

It also did not go through academic peer review. I can only hope that further reviews would have identified the problems I noted and forced the authors to provide some more clarity, footnotes, &re-consideration into how the information was presented and would be interpreted. 27/n

So, recapping findings and extracting some lessons:
Plastic bags aren't that bad, in many ways, considering the choices, BUT cotton isn't that bad either, particularly if you use it something like 50+ times, and then it can be better in many ways for the environment.
28/n

LCA is highly subjective - go read the primary source & actual study & find out where those numbers come from!
Beware false precision - think in orders of magnitude.
Be numerate - 7000x of a miniscule number can also be a miniscule number. 29/n Good takes:

https://twitter.com/arthurhcyip/status/1115749677746593792

Oh, and make sure you wear your cotton shirts 7000 times, or you'll be in trouble for depleting more ozone than the disposable LDPE that you should have otherwise worn (and thrown away each time)! ;) 30/30

@ale_potenza @zoeschlanger @HankCampbell A thread with comments on the bag LCA that you covered. There were a few important problems and misleading points that you missed. I also partially blame the authors and others who further simplified from your stories and coverage.

@gschivley @xiaojujuliechen @hscottmatthews @stvnyng @furdlog Dear LCA expert friends, did I get anything wrong in this tweetstorm about the cotton vs. plastic bag LCA? Is it common to report the maximum number out of all impact categories in an LCIA?

Screen captures showing the source of the problem

https://twitter.com/arthurhcyip/status/1116017852262354945

Revising this one... Single-use LDPE plastic bags, WHEN 100% disposed, do not compare too poorly against other types of bags, PARTICULARLY if they aren't re-used enough times. BUT we do not have 100% disposal and non-LDPE bags can indeed be re-used many times. 28b/n

I like this take more than mine!

https://twitter.com/arthurhcyip/status/1116075587402108928