You’ve said the headline is “misleading,” but you haven’t identified how. Misleading would mean one of:
a) the number is wrong,
b) the unit conversion is wrong, or
c) essential context is omitted that would change the conclusion.
Which is it?
Using “annual vehicle emissions” as a proxy is a standard way to translate CO2e for lay readers. The headline maps the study’s quantified impact into a familiar unit. That’s not a "mistake". You claiming it to be a mistake is an emotional response, exampled by:
Being less charitable, but likely more realistic, the journalist probably thought framing it in terms of “your asthma is as bad for the environment as road traffic” was a deliberate way of increasing impact by providing an out-of-context statistic to generate more traffic.
What I’m seeing in this thread isn’t a technical objection so much as an emotional one: people who like inhalers don’t like that the impact, when expressed in familiar terms, sounds big. If you think the framing is sensationalist, point to the clause or number that creates a false impression and show the correct one.
If the issue is grammar, name the rule or ambiguity. If it’s sensationalism, show the exaggeration. If it’s “phrasing,” specify the alternative phrasing that would be more accurate and why.
Otherwise, the headline remains both technically accurate and reasonable, and your response is largely an emotional one. Disliking something because it make you feel a way isn’t a rebuttal.