Answer:
"Air weighing 0.075lb/cubic feet which approximates dry air at 70 degrees Fahrenheit and at a barometric pressure of 29.92 in Hg". In SI this translates to a density of 1.2kg/ cubic feet, a temperture of 21 degrees Celsius, and at a barometric pressure of 101.3 kPa.
Reference: Fundamentals of HVAC/R (Carter Stanfield and David Skaves - 2010). Page 37, Top Right Column, 5th line.