The building envelope needs to be complete before the test is carried out. If there are any unsealed service penetrations, for example, this will increase the leakage.
The situation is different in rooms such as hospital isolation wards, which are pressurised or depressurised relative to their surroundings. Airtightness testing of these rooms (and any resulting remedial work) should be carried out prior to commissioning. However, the overall airtightness test for the building can be carried out any time after the building envelope has been completed.
Q. What is white rust?
A. Galvanising is the process of coating steel components with a thin layer of zinc. Although the zinc is relatively reactive compared to other metals, the surface gradually oxidises in atmospheric air to form a passivation layer of zinc oxide to inhibit further oxidation.
If the underlying steel is exposed at a cut edge then the zinc will preferentially protect the exposed steel by sacrificial oxidation. Problems arise when the freshly galvanised surface is persistently wetted with soft water (condensation or rainwater). In this case the zinc reacts to form mixture of zinc oxide, hydroxide and carbonate - white rust. This can occur due to water ingress to galvanised products wrapped in plastic packaging or water ingress between layers of stacked galvanised material.
If the zinc surface is fully exposed to the atmosphere, and can immediately dry out after wetting, then visible damage is much less likely. White rust in services occasionally occurs due to weather ingress to ductwork systems and after humidifier sections in an air-handling unit.
The damage is usually worse than it looks as the volume of white rust formed is very much greater than the loss of underlying metal. However, where possible the white rust should be removed to inhibit further deterioration.
Mechanical removal (wire brushing) is normally adequate to remove the white rust. Over-painting with a zinc-rich paint can restore the appearance and level of corrosion protection.
Chemical removal of white rust is not usually recommended for installed services components due to the difficulty of rinsing the surface.
Q. How frequently do outbreaks of Legionnaires disease occur in the UK?
A. The Health Protection Agency publishes statistics of various infections.
In 2010 there were 12 clusters or outbreaks of Legionnaires disease in the community (as opposed to hospital or travel-acquired infections). An outbreak is formally defined as "two or more cases where the onset of illness is closely linked in time (weeks rather than months) and where there is epidemiological evidence of a common source of infection".
There is a very good article on the statistics at www.hpa.org.uk/hpr/archives/Infections/2012/respiratory12.htm.
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