Prof. PETER BRIMBLECOMBE

University of East Anglia

THRESHOLDS AND DAMAGE

The interest in air pollution in museums has often focused on measuring the concentration of pollutants rather than addressing the damage that they cause. If we examine the damage caused then we become aware of which pollutants should be of principal concern. An interest in damage raises the issue of the "health of collections" and an analogy with medicine even though human populations and the museum collections offer some interesting distinctions.

HUMAN POPULATION	MUSEUM COLLECTION
Same species, but genetically diverse with different health status	Different materials in different conditions
LIFE EXPECTANCY 80 years	Long term for most objects
DAMAGE Chronic and acute	Cumulative (from air pollution)
SETTING STANDARDS - ETHICS Consider sensitive individuals	Sensitivity, uniqueness, value
THRESHOLDS Depends on toxin	Exist thermodynamically, but practically improbable
STANDARDS Concentration - in reality : exposure or dose	Usually concentration , but cumulative flux is best

Thresholds are required by the laws of thermodynamics for the damage of solid objects by gases, but these would set up the allowable concentration of pollutants at impossibly low values. Kinetic thresholds are also possible where reactions are exceedingly slow.

In a practical sense we should consider operational thresholds. This can be thought of in two ways :

1. We could consider this as the concentration where the rate of damage from the pollutant becomes less significant than damage by other mechanisms.
2. Alternatively, it could be the concentration where survival time is sufficient.

The question of standards raises the issue of units. These can be as concentrations with a specified time interval, but this could be represented more explicitly by dose concentrations. In museums, concentrations are often measured but expressing these as a dose parameter would continually remind the conservator that cumulative flux causes the damage not concentration. Dose-flux conversions require thought about deposition velocity. Where possible, gas concentration should be expressed in dimensionless units such as the mole fraction, but more commonly represented as ppb (by volume in the gas phase). Mass units lead to problems because the formula must be expressed.

ADDITIONAL NOTES

Types of damage
Pollution-induced damage is only one type of threat to an artefact. All must be considered when dealing with the protection of objects including;
EROSION OF OBJECTS : Polishing, dust, air and water movement all follow a dose law. With solids, erosion causes loss of surface detail, scratches etc.
BIODEGRADATION : multiple variables
FREEZING : induces physical stress
SOILING : particle deposition
CHEMICAL TRANSFORMATION STRESS : Pollutants, Dose-solid reaction with large volume changes (for example sulphation of stone).

Peter continuted to drive home the message that dose is the KEY, not concentration, as we need to consider the 'concentration multiplied by the time' parameter. Also need to consider the aerodynamic deposition velocity which is related to turbulent air processes and the surface of the objects. The product of both parameters is given the term cumulative flux. Need to integrate concentration verses time curve to get the cumulative results of the pollution deposition. We might also want to consider thermodynamic thresholds.

Finally, questions, or issues, that need to be tackled were raised :

1. Should standards/guidelines be listed as concentration or dose ?
2. Is concentration an instantaneous measurement ?
3. Should spatially and temporal variables be considered ?
4. Don't forget to consider the deposition velocity may be different for different materials.