The Math Behind This

This equation represents the concentration of PM_2.5 particles (µg/m³) C given an AQI I. Where C_h is the concentration of PM_2.5 particles in the upper limit of the current tier and C_l is the concentration of PM_2.5 particles in the lower limit of the current tier. Likewise, I_h is the maximum AQI for the current tier and I_l is the mininum AQI for the current tier.
These tiers can be found at the EPAs website.
e.g. if your AQI is 115, I_l is 101 and I_h is 150. Furthermore, C_l is 35.5 and C_h is 55.4. Therefore, your concentration is 41.1 µg/m³. According to Berkeley Earth, a cigarette is equivalent to 22 µg/m³ of pollution. Therefore, 41.1 µg/m³ is equal to about 1.87 cigarettes. (41.1µg/m³ / 22 = 1.87).