If rainwater is used for drinking, it is often filtered first. Filtration (such as reverse osmosis, ultraviolet sterilization, or ultrafiltration) may remove pathogens. While rain water is pure it may become contaminated during collection or by collection of particulate matter in the air as it falls. While rain water does not contain chlorine, contamination from airborne pollutants, which settles onto rooftops, may be a risk in urban or industrial areas. Many water suppliers and health authorities, such as the New South Wales Department of Health, do not advise using rainwater for drinking when there is an alternative mains water supply available. However, reports of illness associated with rainwater tanks are relatively infrequent, and public health studies in South Australia (the Australian state with the highest rainwater usage rate) have not identified a correlation. Rainwater is generally considered fit to drink if it smells, tastes and looks fine; However some pathogens, chemical contamination and sub-micrometre suspended metal may produce neither smell nor taste and may not be visible.