<Dl> <Dd> a w ≡ l w x w (\ displaystyle a_ (w) \ equiv l_ (w) x_ (w)) </Dd> </Dl> <Dd> a w ≡ l w x w (\ displaystyle a_ (w) \ equiv l_ (w) x_ (w)) </Dd> <P> where l is the activity coefficient of water and x is the mole fraction of water in the aqueous fraction . </P> <P> Relationship to relative humidity: The relative humidity of air in equilibrium with a sample is called the Equilibrium Relative Humidity (ERH). </P>

Classification of microorganisms on the basis of water activity