Pakistan is #2 and the "second most polluted" -- so extending that, USA is 84th on that list and hence the "84th most polluted", with country #106 being Puerto Rico, which is the "106th most polluted", and therefore the least polluted as per that list. So if you must reframe it as "least polluted", then USA is the 23rd least polluted on that list.
First, there are ~200 countries, not 106. Second, Puerto Rico IS the USA. Third, "84th most polluted" out of a list of 106, is just a weird way to talk. It's like saying the valedictorian of a class of 100 is the 100th most underachieving student.
The US, mind you, is not helped by the forest fires of the last few years in this ranking. They really skew things in California and the US as a whole. These forest fires though can be attributed somehow to the building of the interstate system and therefore sort of does add to the wider point.
Europe is pretty bad because of the diesel engine cars. If you change that, air quality will improve dramatically.
Not to detract from your wider point, but this just isn't true. Here's at least one link that counters with data: https://www.iqair.com/world-most-polluted-countries -- the USA is 84th "most polluted" in terms of Air Quality Index on that list of 106 countries.
Also just anecdotally, nobody that's been to both, say, Norway, and the USA can possibly think that the air quality in the two countries is even close!