Air quality and pollutants

Outdoor pollution gets the attention, and in a city centre it matters. But the pollutants that most often damage collections are generated inside the enclosure that was built to protect them. A well-sealed case made of the wrong board is a small reaction vessel, and the object is the reagent.

Where the damage comes from

Organic acids. Wood, chipboard, MDF and many adhesives release acetic and formic acid, and they carry on doing so for years. In an open room this is negligible. In a sealed case with a low air exchange rate the concentration builds. Lead is exquisitely sensitive and develops a white corrosion product; shell, calcareous stone and some glasses are also at risk. This is the single most common self-inflicted pollutant problem in collections.

Reduced sulphur compounds. Hydrogen sulphide and carbonyl sulphide tarnish silver and copper alloys. Sources include wool felt, some rubbers and adhesives, and outdoor air. Silver that needs polishing every few months is telling you something about its enclosure.

Oxidants. Ozone and nitrogen oxides come mostly from outside and attack dyes, rubber and some pigments. Ozone in particular is destroyed quickly by contact with surfaces, so indoor concentrations are usually much lower than outdoor ones, which is one of the few pieces of good news here.

Particulates. Dust is abrasive when it is moved, it is hygroscopic so it holds moisture against a surface, and it provides nutrients for mould and insects. Soiling is also, in practice, the reason many objects get handled and cleaned, and handling is its own risk. Dust and filtration covers the practical side.

Finding out what you have

Direct measurement of gaseous pollutants at collection-relevant concentrations is possible but not cheap. Passive diffusion samplers sent away for analysis give a quantitative answer per pollutant and are the right tool when you need a number for a report or a specification.

For most decisions, though, the question is not what the concentration is but whether a material is safe to put next to an object. That is what the Oddy test answers. It exposes metal coupons of silver, copper and lead to the test material at elevated temperature and humidity for four weeks, and the corrosion on the coupons indicates the risk. It is slow, it is qualitative, and it remains the standard because nothing quicker is as reliable.

Continuous VOC monitoring with electronic sensors has become affordable, but the sensors respond to total volatile organic compounds rather than identifying species, and the relationship between a total VOC reading and actual risk to an object is loose. It is useful for spotting a change or a source, not for compliance.

Reducing it

The order of preference is the same as everywhere else in this field: avoid, then isolate, then remove.

Avoid. Choose case and storage materials that have passed testing. Powder-coated steel, anodised aluminium, glass, and inert plastics such as polyethylene, polypropylene and polyester are the usual safe list. Untested wood products inside a sealed case are the usual mistake.

Isolate. If a wooden case cannot be replaced, sealing the interior surfaces reduces emission substantially, though few coatings stop it entirely and the coating itself needs testing. Barrier films are an alternative.

Remove. Pollutant sorbents inside a case will take up organic acids and sulphur compounds. Activated carbon is broad-spectrum; impregnated aluminas target acids specifically. They have a finite capacity, so they are a maintenance item with a replacement schedule, not a permanent fix.

Ventilate, carefully. Increasing the air exchange in a case dilutes internally generated pollutants but also destroys any humidity buffering the case provided. This is a genuine trade-off and which way it falls depends on whether your objects are more at risk from acids or from humidity swings.

Where this connects

Metals are the material group where pollutants dominate the risk picture, so metals is the natural next page. If you are specifying a new case, display cases covers construction, and acid-free boxes and tissue covers the equivalent question for storage enclosures.