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and digesting whatever they are growing
on in order to survive. There are moulds
that can grow on wood, paper, carpet, and
foods. When excessive moisture (a relative
humidity greater than 60%) or water
accumulates indoors, mould will grow.
Once established, mould will continue
to thrive even at lower humidities and so
continue to release musty odours. Potential
health effects and symptoms associated with
mould exposures include allergic reactions,
asthma, and other respiratory complaints.
The removal of mould is particularly
hazardous, as when it is disturbed it can
release spores into the air that are easily
inhaled. There is no practical way to
eliminate all mould and mould spores in
the indoor environment; the way to control
indoor mould growth is to control moisture.
Viruses and bacteria
Bacteria is carried by people, animals,
and soil and plant debris. Viruses are
transmitted through the air by people
and animals with infectious diseases.
Such ailments as SARS, influenza,
measles, tuberculosis and chicken pox are
transmitted through the air.
Dust mites
Humidities above 50% at normal room
comfort temperatures are associated with
adverse health implications for asthma and
allergy sufferers. If the space humidity is
maintained below 50% for an extended
period, the risk of mites can be substantially
reduced.
Exposure in practice
Table 8.2 in CIBSE Guide A (2006) [3]
provides some guideline values for currently
accepted limit values of exposure to
substances – see Figure 1 for part of this list
– the exposure levels are given variously in
parts per million (ppm) and parts per billion
(ppb) depending on their magnitude.
A BRE study in 2002 [8] indicated a wide
range of contaminants present in homes
with a good degree of correlation with
high levels of VOCs such as toluene with
houses that suffered with condensation
– the likelihood being that high levels of
contaminants were present due to poor
or inadequate ventilation strategies. Both
benzene and toluene are constituents of
tobacco smoke and the survey indicated that
their concentrations were strongly related to
smoking in the home.
The levels of benzene were found to
be above the requirements set by the UK
National Air Quality Strategy in more than
50% of homes.
The levels of NOx and CO are affected
differently to those of the VOCs. VOCs are
linked to building materials and furnishings
that provide a constant source of emissions
throughout the year, whereas CO and NO2
are combustion products whose indoor
emissions are mainly activity related and so
more prevalent in the cooler months. They
are also strongly intermittent and will vary
with the use of such things as gas cookers,
paraffin heaters and tobacco.
Increased levels of VOCs – as given off
by some carpets, PVC flooring and floor
varnishes – tend to make the occupants
more critical [9] of their thermal comfort
– presumably they feel discomfort and
can more openly rationalise it in terms of
temperature and humidity. Inevitably, newer
premises are likely to have higher levels of
VOCs due to the materials used, particularly
flooring and wall coverings. However, the
fabric of a house can also act as a collector
(or ‘sink’) for some substances such as NO2
giving potentially lower values inside than
outside. External influences such as roads
and industrial and commercial processes
will lead to higher levels of NO2.
There do not appear to be recently
published UK surveys of IAQ in energy
efficient homes (those complying
with the Level 4, 5 or 6 of the Code for
Sustainable Homes) – there is a need
Bibliography
CIBSE Guide A1, 2006
ASHRAE Fundamentals Handbook 2009, Chapters 10, 11, 12, 13
ASHRAE Standard 62-2010, Ventilation for Acceptable Indoor Air Quality
CIBSE Knowledge Series, KS6, Comfort
CIBSE TM40: 2006, Health issues in building services
WHO guidelines for indoor air quality: selected pollutants, 2010
CPD ProgrammE
for some research. It is likely that areas
susceptible to increased flooding in the
UK will lead to greater potential problems
in IAQ. Standing water and wet materials
can become a breeding ground for
microorganisms: bacteria, mould, and
viruses.
CIBSE 3 recommends that the following
measures, in sequential order, can be
adopted to reduce the exposure of occupants
to airborne contaminants in buildings:
1. Eliminate contaminant(s) at source;
2. Substitute with sources that produce nontoxic
or less malodorous contaminants;
3. Reduce emission rate of substance(s);
4. Segregate occupants from potential
sources of toxic or malodorous
substances;
5. Improve ventilation, for example by local
exhaust (if source of contamination is
local), displacement or dilution; and
6. Provide personal protection.
When contaminants are being produced
within a space, some basic modelling
may be undertaken (most easily using
a spreadsheet) to establish the averaged
levels. ASHRAE Standard 62 [10] provides
extensive guidance on both single and
multi-space ventilation. CIBSE [11] provides
some useful formulae to establish
ventilation requirements to maintain
internally emitted contaminants at
prescribed ‘safe’ levels.
© Tim Dwyer
References
1. Indoor Air Climate and Productivity, Seppanen, Proceedings of 9th International Conference on Indoor Air Quality
and Climate, Beijing September 2005
2. Fanger PO, Thermal Comfort, McGraw Hill, NY 1973
3. CIBSE Guide A, 2006
4. Scotland & Northern Ireland Forum for Environmental Research (SNIFFER), ER12 Final project report (PM 2.5 in the
UK), 2010, www.sniffer.org.uk/
5. Not just ‘a few wisps’: real-time measurement of tobacco smoke at entrances to office buildings, Kaufman et al, Ontario
Tobacco Research Unit, 2010
6. Report of Independent Advisory Group on Ionising Radiation, The Health Protection Agency (HPA), 2010
7. International Commission on Radiological Protection Statement on Radon, ICRP Ref 00/902/09, 2009,
www.icrp.org/docs/ICRP_Statement_on_Radon(November_2009).pdf
8. Coward et al, Indoor air quality in homes in England (Volatile organic compounds), BRE Centre for Safety, Health and
Environment 2002
9. Fang, L G Clausen & PO Fanger 1996 The impact of temperature and humidity on perception and emissions of indoor
air pollutants Indoor Air ’96, Tokyo, 1996
10. ANSI/ASHRAE Standard 62-2010, Ventilation for Acceptable Indoor Air Quality
11. CIBSE TM40: 2006, Health issues in building services
www.cibsejournal.com April 2011 CIBSE Journal 55