This study shows that in most counties there has been a slight decline in the incidence rate for men under 70 years since the mid-1990s, while the rate for men over 70 years has only recently levelled off. For women, there is a possible levelling-off among the middle-aged, but a persistently marked increase in the oldest age group.
There are nevertheless a few counties that stand out in terms of their trends or incidences. We can see that the proportion of daily smokers in the age group 16 – 74 years is largely consistent with the county-wise age-standardised rates of lung cancer for men, while the picture is less clear for women (Figure 4). Unfortunately, we have only limited data on age-specific smoking status and history at the county level.
Data from the Cancer Registry of Norway have been shown to be near-complete and have high validity, thus providing a reliable picture of the real incidence of cancer (10). In some counties and age groups, however, the number of cases may be low, thus rendering the rates vulnerable to random variations. The changes in trends have therefore been interpreted by examining developments over several subsequent periods.
The situation in Oslo is remarkable. The high rates of cancer among men in the capital city levelled off and intersected those of Finnmark county around 1980, and slipped below the national rate during the 1990s. The proportion of smokers in Oslo is now among the country’s lowest. One may speculate whether part of the explanation could be that Oslo has the country’s highest proportion of inhabitants who come from an immigrant background. Immigrants account for approximately 30 per cent of the city’s population. Disregarding immigrants from Western countries, the major proportion hail from countries such as Pakistan, Somalia, Iran, Iraq, Turkey, Morocco, Vietnam and Sri Lanka. Among these countries, only Turkey and Vietnam have rates of lung cancer in men higher than those seen in Norway (11). Unfortunately, we have as yet no statistics showing the patterns of cancer in immigrants in Norway, so we have no certain information on how immigration may have affected the trends in lung cancer, neither in the capital city, nor at the national level.
Lung cancer is a multifactor disease, even though the smoking habits in the population exceed all other factors in importance (1, 2). The most widespread and strongest risk factors in addition to active smoking include radon in indoor air (residential houses and workplaces) and exposure to carcinogenic substances at work. Other factors, such as passive smoking and general air pollution, may also contribute because they affect large segments of the population over a long period of time, but the individual increase in risk is relatively modest.
High levels of radon in residential houses may represent a strong risk factor (12), but most residential houses in Norway have levels below the prevailing intervention limit of 100 Bq/m³ (13). In general, lung cancer associated with radon tends to affect smokers and ex-smokers, and it is therefore assumed that lung cancer linked to radon largely follows the prevalence of smoking habits. This notwithstanding, it should be noted that even medium levels of radon (from 1000 Bq/m³) may be extremely harmful to smokers and may add an amount of risk equal to that of the smoking itself.
Historically, carcinogenic substances in the working environment have tended to primarily affect men. In some studies, the contribution from workplace exposure has been estimated to approximately 20 per cent (attributable proportion, men) in industrialised regions, with a somewhat smaller proportion at the national level (14) – (16). Even in occupations with a high degree of carcinogenic exposure, the effects of smoking remain important (17), and exposure in the workplace is hardly sufficient to explain more than a small proportion of the differences between the counties.
The prevalence of lung cancer in young adults is regarded by many as a reliable and relatively rapid measure to assess changes in smoking habits (18, 19). The results are relatively modest even here, despite the fact that the Norwegian efforts to combat tobacco use for decades have been based on the mantra «what is most important is to prevent children and youth from taking up smoking». Action plans for a tobacco-free Norway have contained numerous references to passive smoking and children and youth, but relatively few to programmes to persuade adults to quit smoking (20) – (22). In line with these efforts, the decline in the number of smokers has been greatest in the younger age groups (9). However, as pointed out by the World Health Organization, efforts targeting children and adolescents will not produce any results until 50 years into the future, making motivation and help for adults to quit smoking a key priority area (23).
Of the nearly 63 000 cases of lung cancer diagnosed in Norway since 1984, we may assume that 80 per cent were avoidable if tobacco smoking could have been eliminated after this causal relationship was discovered 20 – 30 years previously (1). Assuming that 80 per cent of the lung cancer patients have died prematurely, this human cost of tobacco smoking since 1984 amounts to 40 000 premature deaths.