The black dust phenomenon that has plagued residents in Fgura and neighbouring areas was "likely" to be caused by emissions from the Marsa power station, an expert report had concluded in 2000.

The report should have laid the mystery to rest but nine years later the planning authority is conducting yet another study to determine the source of this phenomenon amid renewed complaints by residents of black dust depositing itself on their balconies and rooftops.

A spokesman for the Malta Environment and Planning Authority said a preliminary analysis conducted last year was not enough to definitively conclude that the power station was the main cause.

"The source of coarse dust in the Fgura area results from the combustion of fuel oil," according to the analysis, which insisted that even though the particles collected from Fgura were very similar to dust from the Marsa power station, no heavy metals were found.

Further tests had to be conducted on fresh samples of fine dust, the spokesman added, to determine whether it contained the presence of nickel and vanadium, two heavy metals that would indicate the power station as the main source. "In August, with the help of the Fgura local council, we identified the areas where to place the dust collectors. However, since the volume of black dust seems to fluctuate periodically, we need more time to collect enough volume to make the eventual analysis feasible," the spokesman said.

The fine dust required for the tests could not be collected directly off roofs and balconies, he added, because it would be contaminated with other dust particles.

The 2000 report commissioned by then Environment Minister Francis Zammit Dimech had found traces of nickel and vanadium in dust collected from Fgura.

The study by British environmental company AEA Technology Environment had analysed dust samples from two locations in Fgura, another two locations in Sliema and one location in Dingli.

"The results demonstrate that deposition in Fgura is markedly different to that in Sliema and Dingli and is likely to be influenced by emissions from the nearby power station," the experts had concluded.

The report highlighted Malta's generally dusty atmosphere but it also found marked differences between the dust deposits collected in Fgura and those in Sliema and Dingli.

"Fgura dust was almost always dark grey and brown in colour and often quite coarse, whereas Sliema and Dingli dusts were a light brown colour and usually quite fine in size," the report said, explaining that the Fgura dust had a "higher abundance of non-natural components".

A proportion of the Fgura dust consisted of molten spherical particles, similar in appearance to fly-ash, commonly emitted from power stations.

The larger particle size of the Fgura dust meant it could not be supported in the air for any great length of time, which suggested that it was emitted from a source that was within three kilometres of the locality.

The British experts had also analysed meteorological data, which showed a prevalent westerly wind that would have blown emissions from the Marsa power station east towards Fgura.

The report did put into the question the scientific validity of the data collected from one of the Fgura sites since on one occasion the person on whose roof the apparatus was installed had added dust obtained from neighbouring rooftops to the sample.

Deposition of dust was found to be lower between December and March, which the experts had attributed to rainfall that washed any dust out of the atmosphere.

The 2000 report by the British experts and Mepa's 2008 initial analysis are both available on www.mepa.org.mt.