Parts per Million  ppm
ppm  or parts per million  is commonly used as a unit of concentration.
Parts per million  ppm  is commonly used as a dimensionless measure of small levels (concentrations) of pollutants in air, water, body fluids, etc.
Parts per million is the molar mass, volume or mass ratio between the pollutant component and the solution. ppm is defined as
ppm = 1,000,000 c / s
= 10^{6} c / s (1)
where
c = molar mass, volume or mass of the solute component (mole, m^{3}, ft^{3}, kg, lb_{m} )
s = molar mass, volume or mass of the solution (mole, m^{3}, ft^{3}, kg, lb_{m} )
In the metric system ppm for mass can be expressed in terms of milligram versus kg where
 1 mg/kg = 1 part per million
ppm can be also be expressed as
 1 ppm = 10^{6} = 0.0001 % = 0.001 ‰
 1 000 ppm = 0.1 %
 10 000 ppm = 1%
Download and print ppm vs. percent and mg/L chart
Alternatively  mass related units to measure very small concentration levels:
 ppb  parts per billion (1 / 1,000,000,000 or 10^{9})
 ppt  parts per trillion (1 / 1,000,000,000,000 or 10^{12} )
 ppq  parts per quadrillion (1 / 1,000,000,000,000,000 or 10 ^{ 15 } )
An alternatively mass related unit to measure larger concentration levels is weight percent which can be expressed as
weight percent = 100 m_{c } / m_{s } (2)
ppm vs. Mass per Unit Volume
The concentration of a component can be measured as mass per unit volume  like mg/liter, mg/cm ^{3} etc.
Weight of substance added to one unit volume of water to give one part per million (ppm)
1 ppm
= 2.72 pounds per acrefoot
= 1,233 grams per acrefoot
= 1.233 kilograms per acrefoot
= 0.0283 grams per cubic foot
= 0.0000624 pounds per cubic foot
= 0.0038 grams per US gallon
= 0.058419 grains per US gallon
= 0.07016 grains per Imperial gallon
= 1 milligram per liter (mg/L)
= 1 microlitre ( μL ) per liter
= 0.001 gram per litre
= 8.345 pounds per million gallons of water
Example  Volume Concentration of Carbon Dioxide in Air
The concentration of carbon dioxide in air is aprox. 400 ppm . The volume of carbon dioxide in one 1 m^{3} of air can be calculated by modifying (1) to
c = ppm s / 10^{6}
= (400 ppm) (1 m^{3} ) / 10^{6}
= 0.0004 m^{3} /m^{3}
Percent by Volume
Volume percent can be expressed as volume per unit volume:
percent by volume = 100 v_{c } /(v_{c } + v_{s } ) (3)
where
v_{c } = volume component
v_{s } = volume solvent
Molarity
Molarity is the number of moles of solute (substance of interest  pollution, etc.) dissolved in one liter (volume) of the solution.
Molality
Molality is the number moles of solute divided by kilograms of solvent.
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