&gt;The purpose of plastic bag taxes isn&#x27;t to &quot;price-in&quot; an externality;<p>Maybe, maybe not; I was responding to a poster who was justifying it on that basis.<p>&gt;More generally, the language of economics (&quot;pricing-in externalities&quot;) doesn&#x27;t make sense when discussing irreversible damage to the environment:<p>&gt;When you can&#x27;t repair the damage, the idea of &quot;pricing-in&quot; externalities is just confused thinking. (You can&#x27;t buy a new planet...<p>Well, you can repair the damage from plastic bags.<p>&gt;2. For any consumer behavior that causes irrevesable damage to the environment, the cost is at once too large to quanify<p>If your model contains infinities, it&#x27;s automatically unhelpful; that takes you into Pascal&#x27;s Wager territory.  If the damage is infinitely bad, then it it would justify draconian measures against even <i>trivial</i> risks of too many plastic bags.  But however bad they are, they&#x27;re not infinitely bad.  Such a model would justify arbitrarily high bag fees, not some piddling ten cents with the hope of changing long-term consumer behavior.