Saharan dust transport events reaching the city of Madrid showed strongest effects of daily concentrations of PM10 on case-specific mortality due to organic, respiratory and circulatory causes. Therefore, during Saharan dust-days these effects were larger in the cold season for respiratory causes and in the warm season for circulatory ones.
The distribution of daily concentrations of PM10 reported in our city, with higher levels on Saharan dust intrusion days, are consistent with the increased contribution of particulate matter from the Sahara because of the natural advections of this. Saharan dust intrusions tend to occur more frequently in the warmer months, which agrees with the frequency of synoptic-scale weather situations , mainly characterized by higher temperatures and low concentrations in relative humidity. Similar synoptic behaviour and frequency of Saharan dust intrusions have been described in the study conducted in the Emilia-Romagna region , while in Athens are less frequent and have no clear seasonal pattern .
The effects of PM10 on case specific mortality found in our study agreed with those previously reported in other European cities. The APHEA-2 study reported that an increase in PM10 by 10 μg/m3 was associated with increases of 0.76% in cardiovascular deaths and 0.58% in respiratory deaths . However, only few studies have assessed the effect of Saharan dust events on the association between PM10 and daily mortality, reporting inconsistent findings. Our results, showing an increase in mortality due to organic, respiratory and circulatory causes during Saharan dust days, are similar to those recently found in Rome . However, another study conducted in the Emilia-Romagna region  concluded that Saharan dust outbreaks are an independent risk factor that increases respiratory mortality. On the contrary, a latest study in Athens  did not found effect modification of PM10 on total and case-specific mortality due to dust events. In addition, studies focused on daily mortality related to dust of non-Saharan dust origin, like those in Washington  and Seoul , also fail to find any effect on Saharan dust events.
Currently, some studies have been focus on the effect of coarse-fraction (PM10-2.5). A study conducted in Barcelona  found a stronger effect of PM10-2.5 on daily total mortality during Saharan dust days, as well as the previously cited in Rome  that also found evidence of stronger effects on cardiovascular mortality. However, the health effects of coarse particles are still controversial, since most of the available studies were not able to isolate an independent effect of coarse particles from fine particles . Furthermore, previous studies of African dust size distributions showed that long distant transport reduces the amount of heavier and larger particles in the Saharan air masses, thus increasing the relative contribution of smaller particles . These data suggested than the usual definition of coarse fraction (PM10-2.5) usually adopted in air pollution monitoring also in epidemiological studies should be use with caution as specific marker of Saharan dust transports, which are probably better overall described by PM10-1 . However, we did not consider other PM fractions since from a previous study we did not found differences between Saharan and non-Saharan dust days for PM2.5 , and no PM1 levels were monitored during the study period. Nevertheless, the issue of harmful effects of Saharan dust has recently been addressed by taking PM10 into account, as recently did in the Emilia-Romagna region  and in Athens . Thus, our study is entirely comparable with those in terms of outcome and exposure evaluated, increasing the ongoing evidence on the mortality induced by the influx of Saharan dust in southern Europe.
A major strength of our study is that seasonal dependency, only conducted previously in the study conducted in the Emilia-Romagna region , has also been addressed. We found strongest effects of PM10 in daily mortality by circulatory causes during the warm season, and on respiratory causes during cold season. This confirms the seasonal pattern of case-specific mortality in Madrid, described in earlier studies more than two decades ago . This seasonal dependency was not found in the Emilia-Romagna region . This could indicate that the activity of biological agents in Saharan air masses differs depending on the season and the environmental conditions experienced by air masses travelling from the Sahara to Southern Europe [4, 11]. An interaction with other environmental factors not included in the analyses could be another feasible explanation.
As limitation, we have not addressed for contextual socioeconomic status and other potential modifiers, such as age and sex. However, the study design that would have meant a spatial adjustment had been very different to that conducted in our study . From a pathophysiological point of view there it is well known that the elderly group is most vulnerable to the short-term effects of air pollution on health , we have not considered age groups due to it was not available. Furthermore, as well as in previous studies [10–12], we do not have available data on chemical and biological components or on the copresence of other pollutants in particulate matter during Saharan dust days, so cannot determine specific components that could account for stronger estimated effects on Saharan dust days, or confirm the natural or anthropogenic origin of particulate matter. Only the study in Barcelona  reported data on chemical composition of fine and coarse particles for Saharan dust and non-dust days, concluding that although coarse particles seem to be more hazardous during Saharan dust days, differences in chemical composition did not explain these observations. Inconsistencies in the existing evidence suggests that characteristics associated with the short-term effects of desert dust should be further studied, with special attention to their composition, biological properties, and anthropogenic components that may be associated with the dust particles .
However, the results found in our study jointly with the knowledge of a possible biological mechanism that indicates that particulate material from the desert carry biological material which makes them particularly harmful to health [5, 26, 27], as well as the findings from other Mediterranean cities, such as Barcelona  and Rome , with similar settings like Madrid, make obvious the need to conduct further studies with a similar methodology in Southern Europe.