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IMPACT homepage Introduction to IMPACT How IMPACT works Using IMPACT
HowIMPACT works

The IMPACT models are built around a deposition-based mass balance model put forward by Weschler et al (1989):

Mass balance equation


  • Ci = indoor pollutant concentration
  • Co = outdoor pollutant concentration
  • acr = air exchange rate (hr-1), the overall building ventilation rate
  • A/V = surface area of interior (m2)/ volume (m3) of interior, ratio
  • vd = deposition velocity (m hr-1 or cm s-1), an expression of how well a particular surface takes up a particular pollutant gas

This model assumes that outdoor pollutants, as trace gases in the air, enter a building through the ventilation system and by infiltration through cracks, crevices, window openings.

If the gases were inert they would remain at a constant concentration in the air, without reacting or causing damage. However, by definition, any air pollutant that damages cultural heritage materials, must be reactive. The model therefore assumes that once inside a building the pollutants, through random motion, will come into contact with the interior surfaces of the building, such as walls, floors, ceilings and cultural heritage objects. Depending on the nature of the surface and the reactivity of the pollutant, the pollutant may react there, being chemically transformed and removed from the air. This reaction will also cause a small change, which may be damaging to the surface on which it occurs.

This model assumes that reactions among pollutants in the air (homogeneous reactions) are not significant, compared with reactions at surfaces (heterogeneous reactions). This is a valid assumption for sulphur dioxide, less so for ozone and nitrogen dioxide, which can participate in homogeneous reactions indoors. However these reactions are driven by light and research by the IMPACT partners (Svendby and Henriksen, 2004) has demonstrated that homogeneous reactions do not significantly affect the IMPACT model at the light levels found in cultural heritage buildings.

A second assumption is that once a gas has reacted at a surface it is not subsequently re-emitted. For sulphur dioxide and ozone, reaction at surface is throught to involve complete decomposition and re-emission does not occur. In some circumstances, nitrogen dioxide can be transformed into the relatively inert gas HONO, which is re-emitted to the air (Spicer et al, 1993, De Santis et al 1996), but this is not thought to have any damaging effects on objects.


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Naturally ventilated buildings

Naturally-ventilated buildings

Air-conditioned buildings

Air-conditioned buildings

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Spicer, C. W., Kenny, D. V., Ward, G. F. and Billick, I. H. (1993)
Transformations, lifetimes and sources of NO2, HONO, and HNO3 in indoor environments.
Journal of the Air & Waste Management Association 43: 1479-1485.

De Santis, F., Allegrini, I., Fazio, M. C. and Pasella, D. (1996)
Characterisation of indoor air quality in the Church of San Luigi dei Francesci, Rome, Italy.
International Journal of Environmental Analytical Chemistry 64: 71-81.

Svendby, T. and Henriksen, J. (2004)
Homogeneous reaction of air pollutants: modelling and testing.
In: Innovative Modelling of Museum Pollution and Conservation Thresholds, Final Report, edited by N. Blades, UCL Centre for Sustainable Heritage, 97-114.

Weschler, C. J., Shields, H. C. and Naik, D. V. (1989)
Indoor ozone exposures.
Journal of the Air Pollution Control Association 39 (12): 1562-1568.

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