* 1. Please enter the last four digits of your telephone number.

* 2. Temperature inversions occur when warm air traps cold air in mountain valleys?

* 3. All are characteristics of a time-stratified case-crossover design except:

* 4. PM2.5 is _______?

* 5. The average PM2.5 level on a non-inversion day was 37.4?

* 6. A person with asthma has an increased chance of going to the ER on the 5-7 day of an inversion?