The conversion equations depend on the temperature at which the conversion is wanted (usually about 20 to 25 degrees Celsius).  At an ambient air pressure of 1 atmosphere (101.325 kPa), the general equation is:
and for the reverse conversion:
Notes:
* Pollution regulations in the United States typically reference their pollutant limits to an ambient temperature of 20 to 25 °C as noted above. In most other nations, the reference ambient temperature for pollutant limits may be 0 °C or other values.
* 1 percent by volume = 10,000 ppmv (i.e., parts per million by volume).
* atm = absolute atmospheric pressure in atmospheres
* mol = gram mole