Unit 9 Indoor air quality and

ventilation requirement

Objectives

To

• Recognise the major indoor contaminants, their main sources

and their health impacts;
• Understand the needs of ventilation;
• Appreciate the recommended standards of indoor air quality;
• Recognise typical Sick Building Syndromes, their main causes
and major remedies;
• Understand the principle of species dilution in an enclosure;
• Be able to carry out calculations for ventilation requirement.

Unit contents

• Common indoor contaminants and their major effects on

occupants
• Basic requirements of fresh air in common buildings
• Dilution equation, its development and solutions for some
common cases
• Examples

Additional readings
BS EN 13779: Ventilation for buildings. Performance requirements for
ventilation and air-conditioning systems (London: British Standards
Institution) (2005)

Eastop, T. D. and Watson, W. E. Mechanical services for buildings.

Longman, 1982

Goodfellow, H. D. Advanced design of ventilating systems, Elsevier

Science,

Heriot-Watt University Unit 9-1

Ventilation & Air Conditioning [D11VE]

9.1 Introduction
People are spending more and more time inside buildings. Apart from
the time for rest, sleeping and other house hold activities, additional
time is spent indoors, for work, entertainment, and recreational
activities. The indoor air quality, like thermal comfort that has been
discussed previously, affects occupants’ well being. Moreover, poor
air quality can even cause health problems. Hence ensuring the air
quality of the indoor environment has been one of the main tasks for
environmental design and building services engineering.

This unit provides some essential information regarding indoor air

quality and ventilation principles, the two physical aspects that have
been identified as the most critical factors that influence occupants’
satisfaction of an indoor environment.

9.2 Indoor air quality

9.2.1 Constituency of air

Clean air in a less habitated area is composed of mainly nitrogen

(78%), oxygen (21%) argon (1%) and carbon dioxide (0.03%).

In cities due to human activities, such as chemical plants and traffic

emissions, more elements are added, such as nitric oxide (NO),
nitrogen dioxide (NO2), sulphure dioxide (SO2), sulphur trioxide (SO3)
and carbon monoxide (CO). Apart from these chemical gases, there
are also ambient respirable suspended particulate (RSP). All of these
are considered air pollutants, which can be health hazardous and
degrade the quality of the outdoor air.

To assess the air quality, various indices are used across the world. In
UK, the Pollen Index is an example used in the spring and summer
season in the MET Office’s forecast, as a significant portion of
population have an allergy to pollen. Other indices, like the level of
pollution (ranging from 1 to 10) are also used in UK. In North America,
the Air Quality Index is used, of which the value varies from 0 to 500,
representing the effects on health from good to hazardous.

For individual cases contaminant, concentration is used to quantify the

level of pollution. This variable is used to assess the effectiveness of
ventilation.

9.2.2 Common indoor contaminants

A building is very likely to contain more contaminants or higher levels

Unit 9 - 2 Heriot-Watt University

 Indoor air quality & ventilation

of a specific contaminant than its surrounding outdoor environment.

Therefore replacing indoor stale air by outdoor “fresh” one is to be
either achieved by purpose design and operation or by involuntary
infiltration. Also because human beings normally spend about 90% of
their time inside buildings, indoor air quality is a serious concern to
most of us, particularly occupants and building services engineers

In a building the common contaminants are as follows:

Moisture contents

This is the main product of human activities inside a building. It comes

from human respiration and body evaporation. For example an adult
releases about 0.04 kg per hour moisture. This rate increases, as we
expect, as the activity level rises.

Cooking, washing and other daily activities in kitchen and bathroom

add more moisture into an indoor space. Even combustion gives 0.16
kg kW -1 per hour. Often these are the spaces when ventilation is
needed to exhaust excessive moisture in a residential home.

Moisture also causes an indirect but more serious problem: a damp

indoor environment is ideal for the development of moulds, and
moulds are the sources of various fungi that release substances that
cause allergies. They can produce spores. Inhalation of fungal spores
may result in some diseases like, toxic pneumonitis, hypersensitivity
pneumonitis, tremors, chronic fatigue syndrome, kidney failure, and
even cancer.

CO2

This is mainly from occupants’ respiration, although an open fire can

release CO2 as a by-product of combustion. CO2 can be especially
high in spaces with high occupant density, such as classrooms,
cinemas, theatres and so on.

The effects of carbon dioxide contamination by volume are as follows:

1-2% Continuous exposure leads to headaches and dyspnoea

(breathlessness).
3% Severe headaches.
5% Mental depression.
6% Visual impairment.
10 % Unconsciousness.

Odours

These are normally associated with human daily activities, such as

Heriot-Watt University Unit 9-3
Ventilation & Air Conditioning [D11VE]

cooking, bathroom, body odour and other smells, which cause

unpleasant discomfort rather than health problems.

Formaldehyde

This is a colourless, pungent, and irritating volatile organic compound

(VOC), with formula H2CO, used in the manufacturing and chemical
industries, and as a preservative by anatomists, embalmers, and
pathologists. Potential sources in the home include pressed wood
products such as particleboard or fibreboard, smoking, glues and
adhesives, etc.

This is a chemical that has been very common in modern buildings

due to its extensive applications in various household items from
building materials, such as reconstituted boards, paints such as
finishing foams, wall papers and many others such as insulating
foams.

A typical woodchip board can emit as high as 1.7 mg h-1 m-2

formaldehyde.

Material emission
person, m3 E (mg h-1m-2)
Woodchip board 0.46 ~ 1.69
Compressed cellulose boards 0.17 ~ 0.61
Wallpapers 0 ~ 0.13
Plasterboards 0 ~ 0.28
Curtains 0 .

Table 9.1 Typical formaldehyde emission rate

Particulates

There are various types of particulates inside a building, such as fibres

and dusts. They are small, of which diameters range from 0.1 to
10µm, and floating in the air. Known as airborne, they travel with air
movement. They cause adverse health effects, primarily on the
respiratory and cardiovascular systems.

These particles can come from outside as well as being generated

inside a building. The major outdoor source can be traffic, either by
the emission of engines, or dusts due to wearing off road surfaces, or
both. The other common source can be industrial pollution emissions
from nearby factories.

One particularly harmful particulate is asbestos fibre, from building

materials, particularly from fireproofing in old buildings. Now this type

Unit 9 - 4 Heriot-Watt University

 Indoor air quality & ventilation

of material is prohibited in new building applications, and specialists

are required to deal with those already used in buildings.

Another harmful type of particulates is mites, which are harboured by

bedding, carpets & curtains and those fabric materials in houses,
where warm moist conditions are favourable to their growth.

These indoor contaminants cause adverse health effects mainly in the

human the respiratory and cardiovascular systems. The associated
symptoms take some time to appear. The tiny creatures, mites, are
more problematic, as their effects can occur more severely and
quickly than other particulates. They are a major cause for several
forms of allergic diseases, including hay fever, asthma and eczema.

Radon & VOC

Houses built on landfill can receive volatile organic compounds

(VOCs) from the soil and ground water underneath the buildings. They
enter the building together with soil gas due to depressurisation of the
building interior during winter time, when indoor air is warmer than
outside. They can build up to a high level because windows and
openings are mostly closed during this period of time and ventilation is
low.

Their adverse healthy effects to the occupants are similar to those of

formaldehydes mentioned above.

Odourless and colourless, radon is a radioactive gas that can cause

lung cancer. Indoor radon can come from certain building materials
such bricks or stones. In the UK, indoor radon comes mainly from the
soil or rock underneath buildings in some parts of England.

The effects of all these contaminants to the occupants vary from

discomfort to irritation to eyes or skins. More serious is that they can
be either chemically or biologically harmful to human’s respiratory or
other systems.

9.2.3 Quantifying indoor contaminants

As there are various types of contaminants in an indoor environment,

there is not a single method to quantify their level. Terms like
contaminant level, intensity of generation and contaminant
concentration are all used. However their units are rather different.

For moisture content, the concentration is measured as by kg of

moisture vapour per kg dry air.
Heriot-Watt University Unit 9-5
Ventilation & Air Conditioning [D11VE]

For particulates, the level is very often measured by parts-per-million

(ppm). This is a dimensionless unit denoting relative proportions in
measured quantities; particularly for low-value proportions.
Particulates can also be measured by their weight, using a very small
unit, µg per cubic metre air, (µg m-3).

As a radioactive material, radon is measured by a special unit, Bq m-3.

In UK, an remedial action has to be taken when indoor radon level is
over 200 Bq m-3.

9.2.4 Sick building syndromes and criteria for indoor air

These syndromes experienced by the occupants, particularly in those

fully air conditioned buildings, are associated with the sensation of
stuffy, or dry eyes and eyeaches, or watering eyes and running nose,
irritation of mucous membranes, headaches, lethargy and many
others symptoms. Some severe ones may also have those cold-like
symptoms, such as sore or dry throats and breathing difficulty.

Apart from these medical effects, there are some psychological ones,
such as tiredness, difficulty in concentration, and frequent fatigue. All
of these significantly affect the well being of the occupants and studies
have found that SBS has been responsible for reduced productivity of
organisations who occupy buildings with poor quality indoor physical
environments.

SBS complaints are normally associated with high level of indoor

contaminants. This is for two reasons. First it due to the presence of
indoor contaminants sources, such as building and finishing materials,
or high density of occupants. These types of complaints are common
in buildings with extensive use of carpets and soft furnishing materials,
particularly when cleaning is inadequate and irregular. Secondly it is
because of insufficient ventilation. A typical example is those totally
sealed buildings, which were designed to reduce cold air infiltration
and hence the heating loads. As a result the background infiltration is
low and contaminant concentration builds up.

Studies have revealed that SBS complaints can also come from fully
mechanical ventilated or fully air conditioned buildings. There are
many reasons for this. The most common one is lack of regular
cleaning and maintenance in the ductwork and filter device in the air
handling units. The other could be the discrepancy between the
design target and the actual value for the fresh air supply. Although
the design conditions are sufficient, the actual installation or operation
may not deliver the design ventilation rate.

Absence of day-lighting and lack of outdoors views have also been

Unit 9 - 6 Heriot-Watt University

 Indoor air quality & ventilation

identified as two critical causes for SBS. These two problems are
common in buildings with large span and deep plan layout, therefore
contribute to SBS.

9.3 Ventilation requirement

The provision of a ventilation air supply to both occupied and
unoccupied spaces within buildings and other inhabited structures,
such as forms of transport, is necessary in order to:

• Replenish the oxygen supply.

• Dilute carbon dioxide odours, and process emissions.
• Prevent the build up of potentially explosive vapour mixtures in
unoccupied plant spaces.
• Provide air movement, as a constituent part of comfort, and control
airborne contamination in industrial ventilation.

While the detailed treatment of ventilation is outside the scope of this

text, the basic techniques and mechanisms are clearly an applications
of fluid mechanics principles.

There are two techniques available to provide space ventilation,

namely natural ventilation, relying upon differential pressures or stack
effects acting on “gaps”, either intentional, such as window openings,
or unintentional, such as cracks in structures that allow an air
infiltration path; and mechanical ventilation, relying on fan driven
systems operating within sealed structures, e.g. aircraft at altitude,
submarines or buildings constructed to have low infiltration
characteristics.

It is clear that the first is suitable for domestic buildings and structures
where neither close energy management nor process-generated
contamination is considered a problem.

The second mechanical technique is suitable for energy controlled

buildings, those where sealing the building becomes necessary for
other reasons, e.g. acoustic pollution, and buildings incorporating
processes generating contaminants. Mechanical ventilation is
naturally the only alternative for the other examples quoted above.

9.3.1 Quantifying ventilation

Ventilation in buildings is quantified in a couple of ways, including

fresh air supply, room air change rate, and ventilation effectiveness.

Heriot-Watt University Unit 9-7

Ventilation & Air Conditioning [D11VE]

Fresh air supply rate, Q , normally out door air supply rate, is simply
the total volumetric flow rate of the outdoor air into the room, with a
unit of Litre per second (L s-1). This variable is used in British
Standards to specify a basic requirement of fresh air in a specific
indoor space, for either passive ventilation, including both infiltration
and natural ventilation or for mechanical ventilation.

Air change rate, n, is a measure quantifying how many times the

indoor air has been completely replaced by the outdoor air within one
hour duration. Its unit is air changes per hour (ACH). When the air
change is due to the uncontrollable infiltration, this rate is noted as
Infiltration Rate.

Conversion between the two key variables:

V (m 3 ) × 1000 × n(ACH)
Q(l/s) =
3600

Or
Q(l/s) × 3600
n( ACH) =
V (m3 ) × 1000

where V is the net volume of the space (the units are within the
brackets).

The use of litres/s per person is best if the occupation of the space is
likely to be high or varying. The use of litres/s per square metre of
floor space is best for situations where the occupancy is fixed, e.g. an
office space. The use of air changes per hour can be misleading in
spaces with a high ceiling and hence a large volume.

9.3.2 Basic requirement

Minimum ventilation rates are provided as guidance in the design of

building services systems and are normally based on the need to
dilute carbon dioxide and odours, provide air movement and/or reduce
heat loads. Specialist minimum rates for the prevention of process
contamination or explosive mixture build up are also available. Some
general guidelines applicable to habitable spaces are indicated in
tables 9.2 and 9.3. The supply of ventilation air flows to achieve these
levels can be either natural, i.e. relying upon cracks, windows, doors
etc, or mechanical involving a fan and ductwork network.

Unit 9 - 8 Heriot-Watt University

 Indoor air quality & ventilation

Air space per Fresh air supply per

person, m3 person, litre/s. (air changes/h.)

Minimum Non Smoking Smoking

3 11.3 17.0 (2.0) 22.6
6 7.1 10.7 (6.5) 14.2
9 5.2 7.8 (3.2) 10.4
12 4.0 6.0 (1.8) 8.0

Table 9.2 Guidelines on minimum ventilation rates

Application litres/s/person litres/s/m2

Domestic 8 - 12 ---
Boardrooms 18 - 25 ---
Bars 12 - 18 ---
Department stores 5-8 ---
Factories - 0.8
Garages - 8.0
Operating theatres - 16.0
Hospital wards 8 - 12 ---
General offices 5-8 1.3 - 2.0
Private offices 8 - 12 1.3 - 2.0
Restaurants 12 - 18 ---
Theatres 5-8 --- .

Table 9.2 Levels of ventilation required in various building

applications

(Note - lavatory spaces, particularly those without external openable

windows are a special case where the recommendations are that
mechanical ventilation is provided at a rate equal to 15 air change per
hour, 80 litres/s per wc bowl installed or 16 litres /s per m2, whichever
is the greatest.)

Carbon dioxide is a common contaminant whose concentration builds

up in a space is monitored. Carbon dioxide is present in ambient air at
a level of 0.03 % and is generated by the occupants of a space at a
rate of 4.72 . 103 litres/s.

The concentration equation to be developed will allow the changes in

concentration level in a space of a contaminant to be predicted. It will
be shown that this equation can be applied separately to any number
of contaminants present in a space provided that these contaminants
do not react with each other.

Heriot-Watt University Unit 9-9

Ventilation & Air Conditioning [D11VE]

9.3.2 Calculation of ventilation requirements

Figure 9.1 illustrates the general case of ventilation and contamination

growth, or decay, within a space. Processes or occupation of the
space will cause the contamination levels to rise unless the space is
adequately ventilated. It is also necessary to consider the possibility of
contamination entering the space from outside by means of the
ventilation airflow. It is possible to develop a general expression to
encompass these effects, the non relevant terms being dropped if the
effect they represent is not present.

Figure 9.1 Decay equation - definition of terms

It is usual to refer to the contamination levels as parts per 1,000,000,

known as ppm. Then in the derivation of the general contamination
equation, the following terms may be defined:

ci initial concentration of contaminant in the space at time zero, in

parts per million (ppm).
c concentration at time t, in parts per million (ppm).
ca concentration of the contaminant in the incoming air supply, in
parts per million (ppm).
Q incoming ventilation air supply expressed as a volumetric flow
per person per second, or m3s-1 per person.
Gc volume of contaminant produced per person within the
space, in m3s-1 per person.
V volume of the space per person in occupation, m3 per person
n number of air changes per hour for the whole space.

It is thus possible to write down a balanced equation across a small

time increment dt as follows:

Unit 9 - 10 Heriot-Watt University

 Indoor air quality & ventilation

In a time increment dt, the contamination in the space increases due

to possible inflow via the ventilation air and due to generation within
the space,

 ca 
= Q + G dt per person in the space
 1,000,000 

Contamination leaves the space with the extract air, here it is

assumed that there is no build up of pressure in the space so that no
“air storage” term exists in the continuity of air flow equation.

This removed contamination has a value:

 c 
= Q dt
 1,000,000  per person in the space.

(It will be seen that the extract air flow is assumed to be equal to the
inflow rate, it is the level of contamination carried by this flow that
differs).

The net change in contamination in the space over the small time
increment dt is thus:

 ca   c 
 Q 1,000,000 + G  −  Q 1,000,000 dt
   

Expressed as a concentration dc in parts per 106 of air/unit volume of

room space:

dc  ca   c  1,000,000
=  Q + G  − Q 
dt  1,000,000   1,000,000  V
…………….(9.1)

Rearranging:

dc Q Q 106 G
+ c = ca +
dt V V V ………………………….(9.2)

This differential equation may be solved by use of an integrating factor

as:
eQt /V

Combine the first two terms to form a function capable of integration:

Heriot-Watt University Unit 9-11

Ventilation & Air Conditioning [D11VE]

dc Qt /V Qc
( )
d ceQt / V / dt = eQt / V
dt
+e
V
…………………………(9.3)
Qt / V  dc Qc 
=e  + 
 dt V 

Qt / V
Hence multiplying both sides by an integrating factor e allows the
solution of the differential equation:

 dc Q  Q 106 G 
eQt /V  + c  = eQt /V  c a + 
 dt V  V V 

combine this with (9.3)

d (ceQt /V ) Qt /V  dc Q  Q 106 G 
= e  + c  = eQt /V  c a + 
dt  dt V  V V 

Q 106 G 
d (ceQt /V ) = eQt /V  c a + dt
V V 

Q 106 G 
d (ceQt /V ) = eQt /V  c a + dt
V V 

Integrating both sides yields:

Q 106 G  V
ceQt /V = C + eQt /V  c a + 
V V  Q

 10 6 G 
ce Qt / V
=C +e Qt / V
 ca + 
 Q 

At t=0, c = ci hence C is calculated as:

 106 G  10 6 G
ci = C +  ca +  and C = ci − ca −
 Q  Q

 10 6 G  −Qt / V  10 6 G 
hence c = ci e −Qt / V −  ca + e +  ca + 
 Q   Q 

Thus a general expression for the contamination within the space at

any time t is given by:

Unit 9 - 12 Heriot-Watt University

 Indoor air quality & ventilation

 106 G 
c = ci e −Qt /V + (1 − e −Qt /V ) ca + 
 Q 
…………………………..(9.4)

Two points are worth reinforcing at this stage:

1. The contamination equation is only valid during continuous

processes. If there is a change in the applied ventilation rate or
in the rate of production of contaminant by a process within the
space then the analysis must be restarted with a “new” time
zero. Naturally the final contamination concentration becomes
the initial value for the new application of the concentration
equation.

2. The equation may be applied simultaneously and in parallel to

two or more contaminants within the same space so long as
these contaminants are separate entities and do not react with
each other.

A number of special cases may be considered:

1. Fresh air supplied to the space contains no contaminant, e.g. if

the contaminant is only present in the room as the result of a
process or as a means of monitoring ventilation, and there are
no people or processes active in the space, thus

ca = 0 and G = 0 and n = Q , (9.4) becomes

V

c = ci e − nt
……………………………………...(9.5)

This is the standard decay equation used to determine the

natural ventilation of a space. If a known concentration of tracer
gas is introduced into the space at time zero and the level of
this contaminant is then monitored for a period of time it is
possible to determine the value of the “number of air changes
per hour”,n. Plotting c vs t in a natural log field results in an
equation:

ln c = ln ci − nt ……………………………………
(9.6)

which gives n as the gradient of the resulting straight line. (Note

that t is always in seconds to maintain SI units.) In utilising this
technique to determine the natural ventilation rate it is worth
stressing that efficient mixing of the tracer gas with the air

Heriot-Watt University Unit 9-13

Ventilation & Air Conditioning [D11VE]

within the space is necessary to obtain a realistic measurement

of “air changes per hour”.

2. If the supply is contaminated with a contaminant not initially

present in the space then the level of this contamination will
rise:

G = 0 , ci = 0 and (9.4) becomes

c = ca (1 − e − nt )
…………………………………… (9.7)

3. If the contamination is only a direct result of the people or

processes active in the space then the initial contamination is
zero as is the contamination carried in from outside:

ca = 0 , ci = 0 and (9.4) becomes

10 6 G
c=
Q
( )
1 − e −Qt / V ……………………………….… (9.8)

It follows that any combination of the above processes may be

considered provided that the “clock is stopped” at each time
that the overall process changes, for example a change in the
extract rate or the cessation of a process that generated a
contaminant.

Figure 9.2 illustrates each of the cases above, while figure 9.3
illustrates the change in contamination level due to a process
that is intermittent over a 24 hour period, for example paint
spraying.

Unit 9 - 14 Heriot-Watt University

 Indoor air quality & ventilation

Figure 9.2 Contamination variation under a range of imposed

conditions

Figure 9.3 Variation in contamination concentration over a 24 hour

period during an intermittent period: Example 1

It is essential to remember in using the equations for concentration

build up or decay that the SI system of units must be applied and so
time must be expressed in seconds, although at the beginning air
changes per hour is often used in practise.

Heriot-Watt University Unit 9-15

Ventilation & Air Conditioning [D11VE]

9.4 Example
A garage has a volume of 60 by 30 by 3m and contains cars that
generate 0.0024 m3s-1 of carbon monoxide.

1. Calculate the number of air changes per hour if the garage is in

continuous use and the max permissible concentration of
carbon monoxide is 0.1 %.

2. Calculate the number of air changes per hour if this max. level
is reached after 1 hour and the garage is then out of use.

3. Calculate the concentration after 20 minutes of this 1 hour

period.

4. For case 2 above determine the time necessary to run the

ventilation system at the rate calculated in 2 to reduce the
concentration to 0.001%.

(As no information as to the number of occupants is given assume

one person. It should be clear that the number of occupants will
cancel out of the equation in any case).

Solution
The general expression, equation 1, for contamination at time t within
a continuous process for any individual contaminant is thus:

 106 G 
c = ci e −Qt /V + (1 − e −Qt /V ) ca + 
 Q 
where:

ci: initial concentration of contaminant in the space at time zero.

C: concentration at time, t.
ca: concentration of the contaminant in the incoming air supply.
Q: incoming ventilation air supply expressed as a volumetric flow
per person per second.
G: volume of contaminant produced per person within the
space/sec, m3s-1.
V: volume of the space per person in occupation.
n: number of air changes per hour for the whole space.

In case 1
ca = 0, ci = 0.1 % or 1000 parts/1,000,000 (1000ppm) as the garage
in continuous use.

Unit 9 - 16 Heriot-Watt University

 Indoor air quality & ventilation

V = 5400 m3
G = 0.0024 m3s-1
Q = V n / 3600 to retain seconds as the time unit.

thus,

1000 = ( 0 + 106 * 0.0024 / ( 5400 n / 3600 )) ( 1-e-nt ) + 1000 e-nt

10 = 16 / n (1 - e-nt ) + 10 e-nt
n = 1.6 as in this case of continuous use t would be infinite.

In case 2
ci = 0, c0 = 0 and c = 0.1% at time t = (1 * 3600) secs

 106 G 
c = ci e −Qt /V + (1 − e −Qt /V ) ca + 
 Q 
− nt  106 × 0.0024 
1000 = 0 + (1 − e ) 0 + 
 5400n / 3600 

10 = 16 / n ( 1 - e-n )

n = 1.6 ( 1 - e-nt )

if n = 1.5,
1.5 = 1.6(1 - .22) = 1.24 error = - 0.26

if n = 1.2,
1.2 = 1.6(1 - .30) = 1.112 error = - 0.088

if n = 0.9,
0.9 = 1.6(1 - .41) = 0.95 error = + 0.05

if n = 1.0,
1.0 = 1.6(1 - .37) = 1.01 error = + 0.01

By trial therefore, the necessary value of n = 1.0 air changes per hour.
(A graphical representation of this equation would give a better
approximation.)

Note that this is < Case 1 as the limit is reached earlier.

In case 3, the problem is to calculate c after 20 minutes, 1200

seconds or 0.33 hours. Apply the general equation, 1, as above with
t = 1200:

Heriot-Watt University Unit 9-17

Ventilation & Air Conditioning [D11VE]

 106 G 
c = ci e −Qt /V + (1 − e −Qt /V ) ca + 
 Q 
 106 × 0.0024 
c = (1 − e −1.0×0.33 ) 0 + 
 5400 / 3600 
= 1600(1 − 0.72) = 453 ppm

In case 4, the initial concentration is 0.1 % and no new contamination

is generated, hence G = 0 and as no carbon monoxide is carried in by
the vent system, it follows that ci = 0. The target c is 0.001%, or 10 ppm.

Applying (8.5),

c = ci e-nt

results in

10 = 1000 * e-1.0t

as the final contamination level aimed at is 0.001%, and the air

change rate is 1.0 per hour.

Thus, with t in hours,

1.0 t = ln (100) = 4.61
t = 4.61 hours
Table 9.3 Design criteria for indoor spaces (CISBE Guide A)

Unit 9 - 18 Heriot-Watt University

 Indoor air quality & ventilation

Questions and outline answers

1. List and explain the need for ventilation in a habitable space.

Outline Answer
1 Need for respiration
2 Replacing indoor air with outdoor air
3 Cooling: removing indoor heat gains
4 Extraction, smoke in the event of fire
5 Extraction, excessive moisture
6 Extraction, hazardous chemicals
7 Psychological satisfaction: air movement

2. List the major contaminants in a house and suggest why they

might be undesirable.

Outline answers
1 Odour
2 Carbon dioxide
– respiration
– combustion
3 Tobacco smoke
4 Particulate
5 Water vapour
– respiration
– combustion
– cooking/washing
6 Formaldehyde
– woodchip boards
– compressed cellulose boards
– plasterboards
– wallpapers
– carpets
– Curtains
7 fibres
8 Radon & VOC

Can casue discomfort, poor well being, even illness and consequently
reduced productivity in the organisation who occupy the building.

1 Light discomfort:
– Smell (olfactory system)
– Irritation (eyes, skins)
2 Severe discomfort and sickness feeling due to the harmful, noxious,
harzardous chemical/biological substances:
– Damaging internal organs through respiration;
– Damaging skins, eyes, by direct in contact with the
contamined air....

3. List and explain the factors affecting thermal comfort and

describe methods of evaluating each of these factors when
Heriot-Watt University Unit 9-19
Ventilation & Air Conditioning [D11VE]

attempting to assess the thermal comfort conditions within an

occupied space.

4. Explain how a human body exchanges heat with its surrounding

environment and why an indoor environment can be still
thermally uncomfortable even the heat balance is already
achieved.

5 A classroom having volume of 283 m3. undergoes 1.5 air

changes per hour from natural ventilation. The concentration of
carbon dioxide in the outside air is 0.03 % and the production
of carbon dioxide per person is 4.72 . 10-6 m3s-1.

a. What is the maximum occupancy of the space if the

carbon dioxide concentration is to be less than 0.1 % at
the end of the first hour, assuming an initial
concentration equal to the ambient outside conditions?

b. What is the maximum occupancy if the space is

continuously used and the concentration must never
exceed 0.1%?
(22, 17)

6 If regulations stipulate that the minimum amount of fresh air to

be supplied to a cinema is 8 litres/person/second and that the
minimum amount of space allowable is 12 m3/person, calculate
the concentration of carbon dioxide present after one hour as a
%. Assume that fresh air contains contaminant at 0.03% and
that contaminant generation within the space is 4.72. 10-3 litres
s-1per person.
(0.084%)

7 Use the general equation for the contamination level in an

enclosure to indicate how the natural ventilation for an
unoccupied space may be measured.

Write down the form of the contamination equation that would

apply to the following cases, illustrating your answer with
appropriate contamination vs time curves.

(i) Generation of contaminant within a space that is

mechanically ventilated.

(ii) Decay of an initial contamination level within a space by

mechanical ventilation, followed by a period of natural
ventilation.

(Indicate any assumptions in your answers.)

Unit 9 - 20 Heriot-Watt University

 Indoor air quality & ventilation

A lecture theatre has a volume of 1500 m3. The maximum level

of CO2 at the end of an hour is 0.1%, assuming an initial
concentration of 0.03%, equal to that in the outside air used for
ventilation. Natural ventilation can provide 1.5 air changes per
hour. If each occupant generates CO2 at a rate of 5.0X10-6
m3/s, determine:

a) The maximum number of occupants who can use the

space for an hour.

b) The air change rate that would be necessary to double

this occupancy for an hour.
(113, 3.75)

8 The general equation for contamination decay within a space

may be expressed as:

c = { c0 + 106 Qc / Q } { 1 - e-Qt/V } + ci e-Qt/V

a) Use this general equation to determine the number of air

changes per hour that result from natural ventilation in
an unoccupied space.

b) Write down the special forms of this equation for the

following cases, illustrating the resulting expressions
graphically.

(i) Following a zero initial contamination, a process is

instituted that generates a steady contaminant
input to the ventilated space, incoming air being
free of contamination.

(ii) On completion of a process, contamination is

allowed to decay by steady ventilation of the
space, incoming air again being free of
contamination, initially by mechanical ventilation
and later by natural ventilation.

c) Paint spraying takes place in a ventilated space from

9am to 12 noon and from 2pm to 5pm. Sketch the
variation of paint solvent fume contamination over a 24
hour period, assuming a zero initial contamination at 9
am and a constant ventilation rate from 9am to 9pm.
(Note that no calculations are required but your answer
should reflect changes in conditions in the space during
the 24 hours considered.)

Heriot-Watt University Unit 9-21