IAQ: Relative Humidity

Air expands when heated and contracts when cooled. In the picture to the right, the test tube and the beaker both contain .5 ounce of water. The test tube volume represents dense cool air. The beaker volume represents expanded warm air. In this visual example, the relative humidity of the beaker is 10%. Contrast this the relative humidity of the test tube at 75%.

Relative humidity defined: the amount of water vapor in the air, expressed as a percentage of the maximum amount that the air could hold at the given temperature.

How do relative and absolute humidity differ?

Absolute humidity defined:  a measure of the actual amount of water vapor in the air, regardless of its temperature.

The HVAC industry has used psychrometric charts to values of relative humidity, absolute humidity, and dew point from psychrometer readings. In the two charts to follow, barometric pressure was 29.921″ Hg, altitude was sea level, outdoor temperature was 40.0 OF, outdoor relative humidity was 80%.

Using Carrier psychrometric charts we see how heating the indoor air to 75 OF yields an indoor relative humidity of only 22.4%. As you can see on the charts, the absolute humidity of 29 grains/lb. of dry air remains the same, unaffected by temperature change.

The point of the above exercises is to demonstrate the effect of heating winter air. You will find this useful when discussing the flu season and the importance of maintaining indoor relative humidity for the prevention of infections from airborne pathogens.

Why manage relative humidity?

“For most areas within health-care facilities, the designated comfort range is 30%–60% relative humidity. Relative humidity levels >60%, in addition to being perceived as uncomfortable, promote fungal growth.” CDC Guidelines for Environmental Infection Control in Health-Care Facilities (2003)

Scientific studies reveal how influenza infects when the air is cold and dry or when it is humid and rainy. No one knows all the reasons this is so, but obviously, as the chart to the right shows, 50% relative humidity is close to optimum to prevent influenza and this probably extends to all airborne viruses, regardless of the season.


Published by John White

A lifetime (over 50 years) of experiences with automation and control systems ranging from aerospace navigation, radar, and ordinance delivery systems to the world's first robotic drilling machine for the oil patch, to process-control systems, energy management systems and general problem-solving. At present, my focus is on self-funding HVAC retrofit projects and indoor air quality with a view to preventing infections from airborne pathogens.

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