External
Since the industrial revolution pollution in the atmosphere has steadily risen. This is a direct by-product of burning fuel in industry and transportation. This pollution is in particle and gas form. This airborne 'dirt' is fine enough to travel into buildings, and even into unsealed museum display cases, before it settles on objects and begins to accumulate. This residue will be unsightly if accumulation builds sufficiently. But as products of combustion, these particles will also contain sooty and tarry material. The gas emissions can contain sulphur dioxide, which reacts with the atmosphere to create sulphuric acid, which will cause damage to objects, particularly objects comprised of marble, limestone, paper, cotton, linen, silk, iron and steel.

Internal
Airborne dust comprises of residues of human skin, hair and textile fibres and mineral dust from building works. This material combines with heavier material such as food debris and insect of rodent focusses. This dust cocktail is a threat to objects as it provides a food source for biological pests.

Smoking is to be discouraged inside the museum leads to a build up of tarry residue on the surface of objects.

If the deposit of these pollutants builds up on objects over time it will have to be removed in a cleaning operation. This cleaning action itself can present a threat to the object and can reveal more destruction than was apparent before cleaning took place.

Preventing this dirt build up is desirable and can be achieved with well made display cases and glazed picture frames. But air filtration will be necessary to remove this pollution from the atmosphere to prevent it reaching the objects.

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