Athens is ok
Compared to the 90s it has improved dramatically, not only due to the financial crisis as many mentioned, but more because of the evolution of car technology - catalytic cars etc.
I have lived in other large cities in Europe and I can tell you, you will not see or feel any difference.
The only issue is maybe during the winter, when Athenians start feeling cold at 15 Celsius and light up their fire places. There are days, especially when we have south winds + fire places... then the air is getting a bit "heavy"
All in all Athens is not a polluted city, but rather a dirty city, as people who live here do not respect the city or do not care about keeping the place clean (complete lack of respect and education).