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04

Feb

The Effects of Dry Air on People

Written by Duncan under Humidifiers | No Comments ""

Dry air has significant effects on people and processes. It affects people’s comfort and health and, consequently, their performance at work.  In the UK, low relative humidity in the workplace occurs primarily in the winter and, with increasingly high summer temperatures, is becoming more significant in warmer months, too.  The optimum level of RH for people is around 50%RH.

On a cold, damp day the outside temperature might be 5°C and 80% relative humidity (RH) but as this outside air infiltrates a building it warms and the relative humidity drops.  So, at a typical inside temperature of 20°C the relative humidity of the same air will have dropped to just 30%RH.  This is unpleasantly and unproductively dry.  On colder days, the situation inside will be worse.  When conditions outside are 0°C/80%RH, it will be just 19%RH inside; even when it’s damp outside, it can be drier than the Sahara inside. 

Electrostatic shocks
Perhaps the most obvious dry air effect is electrostatic shocks.  The threshold for the build of electrostatic is 40%RH.  Below this level, the problem rapidly becomes a nuisance. Above this level, the problem is effectively eliminated although, where under floor heating and certain types of carpet are employed, a higher RH of 55% might be required to eradicate the problem.  Correct humidity levels prevent the discomfort of electrostatic shocks.

Visual discomfort
In dry air, the eyes lose moisture to the surroundings, causing drying the surface of the cornea.  A Danish study indicated that 25% of office workers experienced eye irritation several times each week due to dry air.  Contact lens wearers express discomfort that can increase by a factor of five as the humidity drops from 45% to 20%.  Below 30%RH, dust and dirt deposits on lenses increase causing greater irritation.  Accordingly, the Contact Lens Association recommends a minimum ambient RH of 40%. 

Dry mouth, throat and nose
The nose and throat act as a powerful humidifier for inhaled air and mucous helps protect against infection by trapping microbes and dust.  However, the throat starts to dry in conditions below 55%RH at 20°C and 80% of people report nasal stuffiness, dry mouth and throat after as little as 2 hours exposure at 10%RH/24°C after just 2 hours.  Persistently low RH increases susceptibility to infection as the mucosa dry out.  Appropriate humidity levels prevent these problems.

Perception of odours
Relative humidity also affects perception of smell.  Cigarette odour, described as acceptable at 50%RH, is described as objectionable at 30%RH.  So, workplace smells are made more tolerable by a correct level of RH.

Skin discomfort
If the air is too dry the skin, scalp and hair dry and become brittle.  When skin is too dry it can become rough and, in very dry conditions, can crack, exacerbating skin ailments such as eczema and psoriasis.

Perception of warmth
People’s perception of warmth varies with relative humidity.  It is possible to reduce ambient temperature to provide a sensation of warmth when the relative humidity is at a comfortable level, so reducing heating costs.  However, a limit is reached when the RH becomes higher than 70%, when people start to describe conditions as muggy or sultry. 

Stress and absenteeism
In a survey carried out on behalf of Volvic mineral water, most employees rated dry workplaces (35%RH or less) as high stress environments but workplaces with a relative humidity between 40-60% as low stress environments.   Research has shown that absenteeism of 4% at 40%RH rises to 5% at 20%RH.

Summary
The effects of dry air on people vary with temperature and individual sensitivity but, at temperatures between 20-23°C, which are typical of the modern workplace, a comfortable and healthy level of relative humidity is found at around 50%RH.  Little tangible effect is felt, except by people with sensitive eyes, until the relative humidity drops below 40%RH, at which level electrostatic shocks are generated.  Below this level an uncomfortable and unhealthy environment is created.  According to temperature, conditions above 60-70%RH become uncomfortably humid.  An appropriate range of relative humidity for people at work is therefore 40-60%RH.


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