How severe is your asthma? Where in Paris do you plan on living?
Paris is bustling city and like with most cities pollution can be an issue. While the French government are taking steps to reduce the impact of traffic, etc - this doesn't solve the issue short-term.
An idea would be to live on the outskirts of the city and commute in when you need to. It might also be a good idea to contact a local doctor.
Read the Healthcare in Paris page for more info.