The Canterbury District Health Board says silt dust from liquefaction caused by the Christchurch earthquake should be treated as contaminated but is not a serious health risk.
The 22 February quake caused thick silt to be brought to the surface and the dust is being blown around by strong winds.
A report from the Institute of Environmental Science says the silt dust particles are too large to enter the lungs and should not cause problems for people with respiratory diseases such as asthma.
However, the DHB says the dust should be considered contaminated.
People exposed to it should wash their hands and use hand sanitiser.