erature of 20 ± 1 °C and controlled humidity (50 ± 5) % for 48 hours, before being weighted. Filters were weighed pre- and post-collection on analytical balance with a precision of 10 μg. The PM concentrations are shown in μg of pollutant per cubic meter of aspirate air.

A portion of quartz filter (1/2 of filter) was used to obtain the concentration of As, Cd, Ni, Pb, Al, B, Zn, Mg and Sb in PM10. The samples were separated for their subsequent chemical analysis with acid digestion. Later on, it was necessary to determine its elemental contribution in the chemical analysis. In this study there was an attempt to follow the recommendations exposed in the European regulations on determination of heavy metals in samples of particle pollutants. HNO~3~ and H~2~O~2~ were used as reactants ([@bib31]; [@bib21]). The Inductively coupled plasma mass spectrometry (ICP-MS) allowed the determinations. This is one of the methods specified in the [@bib10]. For this purpose, a Hewlett Packard set, model 4500, was used. The median filter blanks concentration (n = 10) was: As (0.01), Ni (0.09), Cd (0.30), Pb (0.82), Al (0.05), Sb (0.04) and median LOD concentration (n = 10) was: As (0.02), Ni (0.05), Cd (0.40), Pb (1.42), Al (0.08), Sb (0.09).

In the other half of the filter made the analysis of the levels of fluoride, chlorite, chloride, nitrite and sulphate in the PM10. For this purpose, soluble fraction of the samples was extracted with ultrapure water and the samples were analyzed by ion chromatography, by means an ion chromatograph Dionex DX-120. The limit of detection (LOD) was 0.5 μg m^−3^ for sulphate, fluoride, chlorite and chloride. LOD was 0.25 μg m^−3^ for nitrate.

2.3. Statistical analysis {#sec2.3}
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The software SPSS 17.0 was used for Statistic analysis of the data. Pearson correlation coefficients were initially calculated between chemical composition (elements and anions analyzed) and the PM10. And then Pearson coefficients were calculated between the different chemical elemen