Preventing Outdoor Pollution Indoors – Building
Airtightness as a Barrier to Pollution Ingress
Outdoor pollution enters buildings through leakage
openings and via the ventilation system. Unfortunately harmful urban
pollutants, such as fine particles and traffic fumes, are the most
difficult to trap by filtration and, hence, enter the building unabated.
Preventive measures include:
- Implementing
clean air regulations;
- Avoiding
the entrainment of pollutants from obvious local sources;
- Reducing
the ventilation rate during transient periods of local pollution.
European and UK outdoor air quality directives are
evolving rapidly (see http://www.mistral.co.uk/cleanair/info.htm),
while the UK CIBSE publication
TM21 “Minimising pollution at air intakes” reviews the role of air
intake location in relation to local sources of pollution.
The Figure on the right illustrates how the building
itself may be used as a temporary barrier to the ingress of outdoor pollution. This
approach is effective provided that:
-
The building is substantially air tight;
-
The source of outdoor pollutant is transient
(e.g. rush hour traffic);
-
The building is not densely occupied or is,
itself, producing indoor pollutants [See Air Quality
Reservoir].
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The above Figure is derived by
applying standard dilution equations (for example, as described in British
Standard 5925:1991) to a 10m2 segment of floor area in a room
with a ceiling height of 2.5m.
For each segment, ventilation rates of
0, 1 and 8 litres/second (L/s) are applied from the onset of increased
outdoor pollution (i.e. at time = 0 seconds).
For a perfectly sealed building, in
which no ventilation is applied, it is clear that the outdoor pollutant
does not penetrate and hence the building remains free of such
pollution.
At a residual ventilation rate of 1
L/s (e.g. typical of natural infiltration in a fairly airtight building)
the indoor pollutant rises to just under 0.15 (i.e. 15%) of the outdoor
value after a period of an hour.
At the 8 L/s rate (i.e. typical of the
ventilation needed for a single occupant) the indoor pollution level
reaches 70% of the outdoor value after 1 hour. |
