It’s not fair to say São Paulo has not done anything to improve its air quality.
Back in 1986 the Proconve scheme was introduced which limits drivers use of their vehicles one day a week based on the license plate number of the car in question. This alone is supposed to have led to a reduction of gaseous pollutants by 94 percent, according to the CETESB (Companhia de Tecnologia de Saneamento Ambiental). During the 90s Brazil also eliminated the lead content in gasoline and in 2003 Promoto was introduced which regulates emissions from motorcycles (motorbikes were heavy pollutants because catalytic converters and other pollution control technology were only legal obligations for larger vehicles).
Today there is discussion on how far sulfur content in petrol can be reduced over the next few years but despite all of this the air quality is poor and when the weather is hot (which is most of the time) smog is especially common.
Sometimes pictures speak louder than words so here are some shots of the city I took last month, you can clearly see three horizontal demarcations which are, from the bottom up: city, smog & sky.