The number of molecules in even small concentrations of pollutants is staggering.  A 22.4-liter volume of gas at standard temperature and pressure contains Avagadro’s number of molecules.  This is 6 × 10²³ molecules, which when written out is 600,000,000,000,000,000,000,000 molecules.  If we breathe about a liter of air per breath, this smaller volume contains about 30,000,000,000,000,000,000,000 molecules.  Consider carbon monoxide's 1-hour lower grade F concentration limit of 11.0 parts per million.  In other words, only about every 90 thousandth molecule of the air is carbon monoxide.  However, even every 90 thousandth molecule of so many air molecules is a huge 300,000,000,000,000,000 pollutant molecules per breath.  To demonstrate how large this number is, it is the number of people on 50 million planet earths!

To put concentration reductions for better grade levels into perspective, consider carbon monoxide's lower grade D concentration limit of 6.4 parts per million, which is only 58% of the grade F limit.  This reduced concentration might be where “sensitive” people clearly notice health effects of poor air quality.  Fifty-eight percent might seem to be a considerable reduction.  However, this still translates to 170,000,000,000,000,000 molecules per breath!  Even after a seemingly large 42 percent reduction in the concentration,

we still breathe in huge numbers of the pollutant molecules.

Similar numbers apply to other gaseous pollutants.  If the air we breathe contained grade D concentrations of any of these, with each breath we could also inhale 500,000,000,000,000 nitrogen dioxide molecules, or 3,000,000,000,000,000 ozone molecules, or 2,000,000,000,000,000 sulphur dioxide molecules!

The concentration limit for lead is in units of micrograms per cubic meter (ug/m³).  To relate this concentration to numbers of lead atoms, consider that a “mole” of a substance contains Avagadro’s number of atoms, and we obtain a mole in an amount of the substance equal to its atomic weight expressed in grams.  The atomic weight of lead is 207, so 207 grams contains 600,000,000,000,000,000,000,000 atoms.  Scaling this down to the lower grade D concentration limit for lead of 0.081 millionths of a gram of lead per cubic meter of air yields 200,000,000,000,000 lead atoms per cubic meter.  A cubic meter of air contains a thousand of our breath-sized 1-liter volumes, which therefore contain 200,000,000,000 lead atoms each.  To grasp the size of this number, it is the number of people on 30 planet earths!