270 likes | 388 Views
The Relevance of the NOAEL concept and related parameters in defining pollution thresholds for cultural heritage collections. Jens Glastrup The National Museum of Denmark. Articles to discuss:. Studies of Lead Corrosion in Acetic Acid Environments(1).
E N D
The Relevance of the NOAEL concept and related parameters in defining pollution thresholds forcultural heritage collections. Jens Glastrup The National Museum of Denmark
Articles to discuss: • Studies of Lead Corrosion in Acetic Acid Environments(1). • Jean Tétreault, Jane Siriois and Eugénie Stamatopoulou • Studies in Conservation 43 (1998) 17-32. • Corrosion of Copper and Lead by Formaldehyde, Formic and Acetic Acid Vapours(2). • Jean Tétreault, Emilio Cano, Maarten van Bommel, David Scott, Megan Dennis, Marie-Geneviève Barthés-Labrousse, Léa Minel and Luc Robbiola • Studies in Conservation 48 (2003) 237-250.
Experiments: • Experimental conditions: • Copper or lead plates, 2.5 (or 2) x 5 cm • Room temperature • Varying RH (I discuss 54%). • Different acetic or formic acid concentrations.
The NOAEL concept(No Observable Adverse Effect Level) • From (1): • Lead: ”As observed in……., when the acetic acid concentration is below 0.43mg/m3(175ppb), insignificant weight gains on lead were detected” • From (2): • Copper: ”A significant weight gain for copper samples is seen when the formic acid concentration is higher than 2ppmv(2000ppb or 3,8mg/m3)
The NOAEL conceptThe figures • The balance minimum detection limit is 0.1 mg • However, in the figure, 1-2 (difference between the ”same” weight is equalling a difference in weighing of 0.5mg, 3-2 is 0.6mg on the balance. • This means that on a 25cm2 plate we cannot see a corrosion layer less than 128nm/year. • This is at least 128 Cu(Ac)2 molecules 1 3 2
Question 1? • Is a minimum detection level of 128nm satisfactory to define a No Observable Adverse Effect?
The NOAEL concept(No Observable Adverse Effect Level) • Tétreault (2003): The NOAEL approach relies on thermodynamic limitations (Brimblecombe 1994) • But then: • If the concentration shall have no effect, the rate of reaction must approach 0 at low concentrations. • Is this true in this case?
What is the rate of reaction?Decreasing with increasing conc.
What is the rate of reaction?Increasing with increasing conc.
Question 2? • Does a “No Observable Adverse Effect Level” exist?
My conclusions: • It is easy to set standards in air: • But is it the best possibility? • NOAEL is dangerous because of the “specific design” parameter. • NOAEL does not exist, or only to a very limited degree • We are the chemists, the readers are the curators, therefore, our standards should be defined with great, great care.
My conclusions 2: • If we continue to • - define thresholds based on NOAEL and NOAED. • we risk to give acceptance to corrosion layers as thick as 1µm/10year. • we risk to give the impression to curators that at certain low concentrations everything is safe. • we “miss the focus”, by pinpointing a concentration in air, but what about highly desorbing/reacting materials in areas with high ventilation rates? As shown above, at lowest concentrations we probably have the the highest reaction rates. • LIMITS IN AIR SHOULD BE BASED ON FLUX FROM SURROUNDING MATERIALS
My conclusions 3: • Future research should focus on: • Establishing specific reaction rates for different materials in contact. • FAST methods to evaluate materials. • Defining maximum desorbing flux from materials.