Navarre Doctor Rosa María Alás Brun has shown in her PhD thesis defended at the Public University of Navarre, that despite very low contamination rates in Pamplona they still have an influence on death rates. After analysing the development between of the 1991-1999 levels of the five most representative contaminants in Pamplona - particles in suspension, sulphur dioxide, oxides of nitrogen, carbon monoxide and ozone -, Rosa María Alás Brun argues that this level of contamination, despite being low (even below that of the minimum levels recommended by the latest directives of the European Union and the World Health Organisation), has an influence on the rate of mortality of the population. In this vein, the various statistical analyses carried out show a relation between the increases in the concentration of particles in suspension and the short-term increase in deaths in persons due to non-external causes: specifically cardiac and respiratory causes. Particles in suspension involve an ample range of substances dispersed in the air, amongst which are found the so-called ´black smoke´ or particles capable of blackening, derived from the incomplete combustion of carbon and hydrocarbons. The increases in the concentration of sulphur dioxide have an influence, in the short-term, on deaths due to circulatory reasons. Moreover, it has to be taken into account that these results are produced despite the fact that the sulphur dioxide level progressively declined during the period analysed, fundamentally, due to increased restrictions on vehicle exhausts and consequent modifications to vehicles, and also to changes in fuel sources with the gradual replacement by natural gas as an energy source. In order to understand these results, explains Rosa María Alás Brun, one has to take into account the short-term effects of atmospheric contamination on mortality that occur in relation to the levels of contamination present in the days prior to death. Specifically, the greatest association regarding mortality is between respiratory causes of death and the retarded effects of medium and minimum concentration of particles. In the opinion of the author, the observation that the atmospheric contamination levels in a small city such as Pamplona - that do not even reach the limits established by official bodies have an influence on mortality rates, could be explained in a number of ways: Firstly, it could be that the limit values of the Directives are too high; secondly, it is possible that, effectively, there is no minimum threshold level where contaminants start to be harmful, but that they are always so at any concentration; or thirdly, the hypothesis could be considered that, amongst all the contaminants there could be one that has not been studied and is responsible for some part of the effects. ,Of the five contaminants analysed in Pamplona, the particles in suspension, sulphur dioxide, oxides of nitrogen and carbon monoxide undergo seasonal variations, with maximum levels in winter and minimum in summer. This seasonal variation is due, according to Rosa María Alás Brun, to the difference of activity in the sources of contamination: In winter, heating systems are used more and traffic is greater; while in summer there is less vehicular traffic and factories shut down for periods. Ozone, however, shows an inverse behaviour, with maximums during the hot months and minimums in the cold ones, given that it is a secondary contaminant. Nothing emits ozone directly, but it is oxides of nitrogen and other volatile particles that are emitted and it is these which, in sunshine, transform into ozone. This is not the ozone of the stratosphere layer, but the ozone found in the troposphere, in the layer we respire, and which is a toxic gas for us. The ozone rates are, moreover, greater in rural zones.