Based on experimental data, the benzene annual average concentration increases from the value of 3.1 mg·m-3 in Copenhagen to 20.7 mg·m-3 in Athens. Several reasons could brought forward to explain this experimental finding, one of which is the difference in meteorological conditions.      A decrease of the average pollution level has been measured experimentally in correspondence with the increase of average wind speed during each sampling campaign. Towns in northern Europe are constantly windy, being subject to the passage of Atlantic atmospheric disturbances, towns in the Mediterranean area on the contrary (Padua, Murcia and Athens) have weather conditions influenced by the persistent anticyclone regime.

Annual average urban pollution as a function of the city latitude, showing the increase from north to south Europe.

Poor ventilation seems to play a determining role in the establishment of high urban contamination levels, as is shown by the differences between two towns under Mediterranean weather regime and two towns under the Atlantic one (wind speed is the average value measured in each campaign).

benzene urban pollution level  annual average increases in  Europe from north to south  in correspondence with the  decrease of the average ventilation

Neverthless, personal exposure and home monitoring data do not reflect the differences observed between northern and southern European towns in urban pollution levels. The ratio between the most polluted town annual average and the least polluted one is equal to 6.7, but the same ratio between personal exposure average values drops to 3.6 and even to 2.7 when indoor home measurements are concerned.

Personal exposure and home pollution level ratios with urban pollution as an annual average for each town.

benzene exposure level of European  citizens is an average doubled  with respect to the average  urban pollution level

Typical profile of benzene daily average concentrations, obtained by a BTX automated analyser. Each hourly value is the average over a whole sampling campaign. Similar profiles have been obtained in all the towns equipped with the same instrumentation, with minor variations in the peak time, depending on local lifestyles.

The experimental data suggest an explanation of the phenomenon. From the detailed analysis of the volunteers' diaries and from the study of the daily profiles of benzene concentrations, one argues that people stays in the town roads, in general, in the times of the day when urban pollution is at its most.      Daily benzene concentrations oscillate between very low values during night and very high ones in the middle of the day and in the evening. Since most people get about in the streets when benzene concentration is 1.5-2.5 times higher than daily average, one can estimate that the actual outdoor exposure is about twice than that calculated basing on the daily urban average concentration and the time spent outdoor.      Neverthless, this contribution is only a fraction of the total.

Average values of benzene concentration measured in the six campaigns, concerning urban pollution, home concentration and personal exposure levels.

By reconstructing the exposure history of the volunteers by means of their diaries, one realises that peoples spend, on European average, 21.6% of their time outdoors (for work, shopping, transportation, spare time activity, etc.), 59.1% of time at home and the remaining 18.3% in indoor places different from home (shools, offices, pubs, restaurants, etc.).      Therefore, the contribution due to staying at home becomes very important.  On an European basis, if we exclude Athens for reasons that will be clearer later, the average pollution level at home has come out to be 1.51 times the urban level.

This experimental finding is surprising since it was intuitively reasonable to suppose that home pollution came from outdoor pollution, and should not have been therefore higher than that.  The value of domestic to urban pollution ratio tends to raise from southern to northern Europe. This peculiar tendency levels out the differences of exposure compared to urban pollution levels: as soon as outdoor exposure ceases, citizens are subject to domestic exposure for a longer time, and it has a worsening effect in Antwerp, Rouen and Copenhagen, and has an improving effect in Athens.      Athen's data are particular and are useful to confirm the great importance of home pollution. While in all of the other towns the volunteers' homes were inside the monitored area, in Athens volunteers have been chosen living in quarters far from it and with less automotive traffic. This choice resulted in measuring a home pollution level a little bit higher than the European average (10.1 mg·m-3 instead of 8.5 mg·m-3) facing the urban pollution level which is more than three times as high as the European average (20.7 mg·m-3 instead of 6.4 mg·m-3). Well, the overall personal exposure level in Athens turned out to be 18.8 mg·m-3, that is only a 40% higher than the European average (13.2 mg·m-3).

European average of exposure levels for some selected citizen categories, compared with the European average urban pollution level (red bar). Vertical lines show the range between minimum and maximum values. It is clear that also the exposure of the so-called non-exposed categories is higher than urban pollution.

    The gathered data, allow us to put forward some hypotheses about the reason why many European towns, expecially the northern ones, suffer from an indoor pollution level higher than the outdoor one. The main source of indoor pollution is the urban one as it is demonstrated by the good overlapping of respective seasonal trends.
    The reason why, in general, indoor pollution is higher than the outdoor one, even if it reflects its seasonal behaviour, might be due to a lack of balance among input from outside and inside removal. In other words, the house itself might act as a flywheel because of the adsorbing power of the surfaces of walls, floors, furniture and various furnishings. The hypothesis is interesting, since the phenomenon is negligible in southern Europe towns while is noteworthy in northern Europe countries. In northern towns moquette, linoleum and wood linings often replace tiling, marble and bare walls typical of southern towns.