06.05 Olfactory comfort

Categories: Building Comfort

Introduction
Olfactory comfort is an individual's perception of air quality within an environment.  Air quality is dependent on many factors, for example:  fresh air ventilation rate, level of internal and external pollution sources and humidity.  For a diagram outlining man's physiological, behavioural and artificial mechanisms for alleviating olfactory discomfort click here.

Poor indoor air quality is a result of a high sensory pollution load within the building.  The sensory pollution load may be found by adding the loads caused by all the pollution sources within the space, such as:

metabolic gases emitted by humans

tobacco smoke

chemicals from materials and furnishings e.g. formaldehyde

combustion products e.g. carbon monoxide

micro-organisms, e.g. bacteria, fungi and viruses

pollutants from external sources, e.g. radon.

Humidity is also related to indoor air quality as high humidity levels within the space can promote moulds and dust mites, whereas low humidity levels can cause dryness and skin irritation.  Normally few problems occur when the relative humidity is between 30% and 70%.

Indoor air quality is maintained by the introduction of fresh air into the space in order to dilute and extract pollutants.  In terms of automated facades, opening the window when the external environment is fairly clean can have a great affect on the quality of the air within the space.

Although various methods exist for measuring various types of air pollutants, only Carbon Dioxide sensors and Relative Humidity sensors are used in building control applications.  Although these sensors can give the system a reasonable appreciation of air quality for a lot of space types, they are unable to give an accurate representation of the occupant's own sensory perceptions.  Therefore it is often advisable to provide an occupant over-ride when designing an automated vent control system.

Satisfactory indoor air quality is required within a building to provide a comfortable and healthy working environment. Poor indoor air quality can lead to building related illnesses, often referred to as Sick Building Syndrome (SBS). A building is defined as having SBS when occupants frequently complain of symptoms such as, nasal blockages and dry throat, headache, rhinitis and lethargy. The perception of poor air quality occurs when a high level of contaminants is present within the air that we breathe. Airborne contaminants and the way we perceive them can be divided into two groups, pollutants and water vapour (humidity). These two categories are both governing factors when determining the amount of fresh air we need to supply to a space, either through the facade or through a mechanical system.
 

Pollutants
One perception of poor indoor air quality is a result of a high sensory pollution load within the building. The sensory pollution load may be found by adding the loads caused by all the pollution sources within the space, such as:

metabolic bi-products, for example, volatile organic compounds, carbon dioxide, moisture, aldehydes, esters, alcohols and bioeffluents, all emitted by humans;

tobacco smoke;

chemicals from materials and furnishings, for example, formaldehyde;

combustion products, for example carbon monoxide;

micro-organisms, for example bacteria, fungi and viruses;

pollutants from external sources, for example radon, landfill gases, combustion products from cars.

Note that the pollution load could come from the fresh air supply to the room and thus the external environment. The concentration of pollutants (G) is measured in olfs, where 1 olf is equivalent to the emission rate of air pollutants from a standard person.
 

Indoor air quality
Indoor air quality can be maintained by the introduction of fresh air to the space in order to dilute and extract pollutants. Perceived air quality can be expressed by the percentage persons dissatisfied (PPD) in a space/building.
 

This image shows the typical percentage persons dissatisfied (PPD) against the ventilation rate (v').

Perceived air quality can also be measured in decipols ( C_i ), where 1 decipol is the perceived air quality in a space with a pollution source of 1 olf and ventilated by 10 litres/sec of clean air (1 decipol = 0.1 olf.s.l^-1).

C_i  =  112 ( 5.98 - ln (PPD) )^-4

This image shows the relationship between these two measures of perceived air quality.  C_i can also be written in terms of air supply rate and pollution concentration to give:

C_i  =  C_o + 10 G  / V'

C_i    perceived air pollution within an enclosure [decipol]
C_o   perceived pollution of outside air [decipol]
G   concentration of pollution in the enclosure [olf]
V'     outdoor air supply rate to the enclosure [ls^-1]

In deriving this equation it has been assumed that a perfect mixing of the supply air with room air occurs, i.e. perfect dilution of the contaminants. In reality this is very rarely the case and the ventilation’s effectiveness varies in different parts of a space. The ventilation effectiveness (e_v) can be defined as the ratio between the pollution concentration in the exhaust air (C_e) and in the breathing zone(C_i):

e_v  =  C_e / C_i

The equation can now be rearranged in terms of the outdoor air supply rate and taking into account the ventilation effectiveness to give:

V'  =  10 G / [ ev ( C_i - C_o )

This equation shows that increasing the fresh air supply into a space results in the reduction of the perceived air pollution levels. This only applies if the perceived air pollution of the external air is less than that of the internal air.

Air quality can be maintained by providing a sufficient rate of ventilation depending on the sensory pollution load of the space, such ventilation rates are quoted in various design handbooks. However, recent concerns over ventilation heat loss in buildings has lead to the control of air supply to a minimum fresh air requirement. This involves measuring the air quality of a space and adjusting the ventilation accordingly.

Many air quality control strategies use carbon dioxide sensors as they are a good indicator of the pollution caused by sedentary humans as carbon dioxide production is proportional to metabolic rate. However, it must be remembered that they cannot sense non-metabolic pollutants and a satisfactory fresh air rate must still be maintained.
 

Water Vapour
Humidity is also related to indoor air quality as high humidity levels within the space can promote moulds and dust mites, whereas low humidity levels can cause dryness and skin irritation. However, few problems occur when the relative humidity is between 30% and 70%.

In addition to poor air quality, high humidity levels can cause human discomfort in hot weather, by the inhibition of sweat evaporation through the skin as well as the formation of condensation in cold weather, on room surfaces.

Normally outdoor air has a lower moisture content than indoor air, particularly in winter when condensation is a concern. Therefore, as with the pollution load, the build up of moisture within the internal environment can be reduced by ventilation.

The volume flow rate of air needed to dissipate a certain amount of moisture, assuming the ventilation effectiveness = 1, is given by:

V'_o  =  (m_g - m_f ) / [ r_o ( w_i - w_o ) ]

V'_o    volume flow rate of outdoor air [m³s^-1]
r_o      density of outdoor air [kg m^-3]
w_o     specific humidity of outdoor air [kg kg^-1]
w_i      specific humidity indoors [kg kg^-1]
m_g    rate of moisture generation within the building [kg s^-1]
m_f   rate of moisture diffusion through the building fabric [kg s^-1]

The specific humidity (moisture content) can be found using the following equation:

w  = f exp [ 1156 - 4030 ( t_a + 235 ) ]

f   relative humidity [fraction]
t_a       air temperature [°C]

The diffusion of water vapour through the building fabric may be obtained using Fick’s Law:

m_f  =  Dp_vS ( A_i / R_i )

R_i        vapour resistance of fabric i [N s kg^-1]
A_i        surface area of fabric i [m²]
Dp_v   difference in vapour pressure between inside and outside [Pa]

Note that due to condensation a small amount of background ventilation is needed during the winter months. This should be taken into account when weather sealing the building.