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1 Department of Community Medicine, Mercer University, Macon, Georgia and 2 School of Public Health, University of Alabama at Birmingham, Birmingham, Alabama
Requests for reprints: Fan Chen, Department of Community Medicine, Mercer University, 1550 College Street, Macon, GA 31207. Phone: 478-301-4095; Fax: 478-301-4095. E-mail: Chen_fd{at}mercer.edu
| Abstract |
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Methods: The temporal and spatial patterns of ADL were compared with the level of nitrogen oxides (NOx) emissions as well as the use of low-tar cigarettes.
Results: Similar increasing trends followed by declining trends were seen in emission levels of NOx and ADL incidence rates. These trends peak about 20 years earlier for NOx levels than for ADL incidence. Incidence rates of ADL in high NOx emission areas were substantially higher than those in low NOx emission areas. Incidence rates of ADL in Black males are about 50% higher than in White males and can be explained by the differences in air quality related to residence site and size.
Conclusions: The descriptive epidemiologic data help generate the hypothesis that long-term exposure to low-dose NOx may play a major role in causing steep increases in past ADL incidence rates. There is an urgent need to conduct further studies to determine whether the association is a causal relationship between long-term, low-dose exposure to NOx and ADL. (Cancer Epidemiol Biomarkers Prev 2007;16(12):2724–9)
| Introduction |
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It has been hypothesized that the increasing trend of adenocarcinoma is mainly due to the dissemination of low-tar filter cigarettes (2-5). Smoke from low-yield filter-tipped cigarettes is inhaled more deeply than smoke from earlier unfiltered cigarettes and releases higher concentrations of nitrosamines. Inhalation transports tobacco-specific carcinogens more distally toward the bronchoalveolar junction where adenocarcinomas often arise. An alternative hypothesis may be the air pollution that stems from industrialization and urbanization. Pope III et al. (6) reported that long-term elevated fine particulate air pollution exposures were associated with significant increases in lung cancer mortality based on data from a prospective study. Similar results were reported recently by Vineis et al. (7) who conducted a nested case-control study in 10 European countries. They found a 30% increase in the risk of developing lung cancer for those who were exposed to nitrogen dioxide (NO2) at levels >30 µg/m3 compared with those who were exposed to NO2 at levels <30 µg/m3. However, neither of these studies stratified lung cancer data by histologic types.
We conducted this study to determine which of these two factors is the major contributor for the sharp increase of ADL. The relationships between the distribution patterns of ADL and the distribution patterns of air pollution or low-tar cigarette consumption provide epidemiologic evidence to support either the air pollution hypothesis or the low-tar cigarette hypothesis.
| Materials and Methods |
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To assess whether the patterns of ADL incidence are associated with the use of low-tar cigarettes (<15 mg/cigarette) in the United States, we estimated the total low-tar cigarette consumption in the United States by year. This consumption was based on the market share of low-tar cigarettes and total cigarette sales in each year. These data were obtained from the Federal Trade Commission (9).
To estimate air pollution, we collected emission data of seven pollutants including particulate10, particulate2.5, nitrogen oxides (NOx), volatile organic components, sulfur dioxide (SO2), lead, and ammonia emission levels from Environmental Protection Agency (EPA; refs. 10, 11). Because the data for particulate2.5 and ammonia are not available before 1990, we did not describe trends for these two components but estimated the trends of the remaining five pollutants.
To compare the long-term changes of ADL incidence, low-tar cigarette consumption, and NOx emissions, we converted the levels for each of them into an index so that we can measure the change in the same scale. The index for different years was calculated using the level of each year divided by its own level in 1985 for ADL incidence, low-tar cigarette consumption, and NOx emission level.
| Results |
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Table 1 describes the comparison of the changes in indexes for ADL incidence, low-tar cigarette consumption, and NOx emissions by year. We selected NOx as an indicator for air pollution because among trends of the five air pollutants, the one with a pattern most similar to the ADL incidence trend is the trend of NOx, which had a 3.7-fold higher increase at the peak in 1980 compared with 1940, leveled off after 1980, and then declined. The CO, SO2, and volatile organic component trends showed only 50% increases, which is much smaller compared with the increase of NOx. These three components declined nearly 30 years earlier than the ADL incidence. No data for Pb level are available until 1970, and there is a substantial decline in Pb emission since 1970. In addition to the similarity in trend pattern with ADL, the total emissions of NOx reflect the level of urbanization and industrialization in the past decade because the major source of NOx is combustion from vehicles and industrial activity. Therefore, we used NOx as indicator of air pollution, which includes nitrogen dioxide (NO2), nitrogen oxide (NO), and possibly small amounts of other compounds of nitrogen and oxygen. The harmful part of NOx is NO2.
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Figure 3 shows that the ADL incidence in high NOx emission areas is much higher than in NOx low emission areas. We cannot contribute the difference directly to the difference in levels of NOx because Utah and New Mexico are both characterized as the low NOx emission areas and low smoking prevalence areas. Utah and New Mexico were ranked the 1st and 14th lowest smoking prevalence state in the United States (15). However, we can estimate the potential effect of difference in smoking prevalence by comparing the squamous cell lung carcinoma incidence because the squamous cell lung carcinoma is well known to have high relative risk due to smoking. The relative risks of cigarette smoking for squamous cell lung carcinoma and ADL are 18.9 and 4.8, respectively (16); therefore, we can expect that if the difference in incidence between high and low exposure areas is due to higher smoking rate alone, the difference would be much greater for squamous cell lung carcinoma than for ADL. However, the data show the opposite. We reviewed the difference shown in Fig. 3 and compared it with that of squamous cell lung carcinoma data, and found that the corresponding difference for squamous cell lung carcinoma is significantly smaller than that for ADL (P < 0.0005). This fact suggests that the large difference in ADL incidence between these two areas is not solely due to smoking. There should be a strong factor besides smoking that is different in these two areas.
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| Discusson |
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One hypothesis explaining the change in ADL incidence in the last half of the century is combustion-source ambient air pollution. We reviewed the temporal trends for particulate10, NOx, volatile organic component, SO2, and lead emission levels. The temporal trend of total NOx emissions, an increase followed by a decline, was pretty similar to the temporal trend of ADL, but about 20 years earlier than the changes in ADL incidence. Twenty years is a reasonable time for an ADL induction period. We did not find any other components that have similar temporal trend. NOx form when fuel is burned at high temperatures, as in a combustion process. The primary manmade sources of NOx are motor vehicles (55%), electric utilities (22%), and the rest is from other industrial, commercial, and residential sources that burn fuels (18). Since the enactment of Clean Air Act in the United States in 1970 and the amendment of the Clean Air Act in 1990, the NOx emission by automobiles and industry has leveled off and then substantially declined.
In contrast, the temporal trend argues against the low-tar cigarette hypothesis for two reasons. First, increases in ADL occurred before the nationwide use of low-tar cigarettes. Increases in ADL incidence started in 1950, but the sales-weighted average tar level was still >37 mg in 1956 (19). The market share of cigarettes with tar <15 mg was only 3.6% in 1970 (20), but about half of the total percent increase in ADL had been completed at that time. Second, there is no downward trend in consumption of low-tar cigarettes before the declining trend of ADL incidence rates that started in 1998.
In addition to the temporal trend, we checked the distribution pattern of ADL incidence by place and population. Nearly all the metropolitan areas in SEER program showed high NOx emissions in EPA data (21), except for the states of Utah and New Mexico, where low NOx emissions were found. To remove the influence of low cigarette smoking rate in low NOx emission areas, we compared the incidence ratio of high to low NOx areas for ADL and squamous cell lung carcinoma separately. It is reasonable to expect that the difference in incidence of squamous cell lung carcinoma between high and low exposure areas would be much larger than the corresponding difference for ADL because the relative risk of smoking for squamous cell lung carcinoma is 18.9, which is much higher than the relative risk for ADL, which is only 4.8. However, the data showed the opposite, which indicates that a strong factor other than smoking is involved in high NOx emissions areas that can explain the unexpected high ADL incidence. Most likely, this factor is air pollution represented by high NOx emission levels. We found that lung cancer incidence rates of 44 states in 1960 are positively correlated to the vehicle density in these states in 1950 (data are not reported here). These data also support the NOx hypothesis because automobile exhaustion is the number one source of NOx emission.
The air pollution hypothesis may also explain the racial difference. Blacks are more likely than Whites to be living in census tracts with higher total modeled air toxics in every large metropolitan area in the United States (22). Moreover, the indoor levels of NO2 are often higher than the outdoor levels, and the NO2 level is related to the size of the house (23). For a house bigger than 2,500 ft2, the highest level of NO2 is 30 ppb, but for a house smaller than 1,000 ft2, the highest level of NO2 is 170 ppb. The percent of families below poverty level is 19.3% and 7.1% for Black alone and White alone, respectively (24); therefore, we can believe the percentage of Black families with residences smaller than 1,000 ft2 is greater than that of Whites, and therefore more Blacks may exposed to higher NO2 level than Whites do. Moreover, residence block groups in the lowest quartile of median family income were thrice more likely to have high traffic density than block groups in the highest income quartile (25). This discrepancy in exposure to air toxins supports the air pollution hypothesis and may explain or partly explain why the incidence of ADL in Black males was much higher than in White males. In contrast, the hypothesis of dissemination of low-tar cigarettes cannot explain why ADL incidence is much higher in Blacks. According to the data in Surgeon General Report (1981), "White smokers choose lower tar products approximately twice as frequently as do Black smokers of the same sex" (26).
The percent of ADL of all the histologic types of lung cancer among women is remarkably higher than among men. In addition to differences in genetic background, the NOx hypothesis may help to explain different proportions by gender because indoor NOx is produced by fuel burning stoves, furnaces, fireplaces, heaters, water heaters, and dryers and all other combustion appliances. If we consider both cigarette smoking and indoor air pollution as causes, men have higher level of cigarette smoking exposure but women may have relatively higher level of exposure to this indoor air pollution compared with men over the past several decades.
Another possible explanation for the ADL trend may be environmental tobacco smoking (ETS). In recent years, stricter regulations have been enacted that reduced ETS exposure. These changes may partially explain the recent down trend of ADL incidence. However, based on the nationwide Current Population Survey, only four states reported smoke-free public areas in 1992, and 32 states did in 1995 (27). If the policies restricting ETS were enacted in most states during 1990s, it is then less likely to be the cause of downward trend of ADL after in 1998 because the years between the policies and the downward trends are too short for the expected induction period. Moreover, the total lung cancer cases attributed to ETS is about 3,000/y in the United States, which accounts for <2% of the 172,570 new lung cancer cases a year (28). If the decline in the number of lung cancer cases due to declined exposure to ETS is 5% per year, we would expect <0.08% changes in total number of lung cancer cases per year. This number is therefore too small to explain the remarkable change in ADL incidence. In fact, for the period 1998 to 2003, the incidence rate of ADL declined by 14% for men and 8% for women. Such large reductions could not be explained by a decline in the number of people who were exposed to ETS. However, ETS may play a relatively minor role in causing ADL.
Other reported possible causes for ADL include exposure to oil during high-temperature cooking in Asia (29-31) as well as wood smoke exposure in Mexican women (32).
Further evidence supporting the NOx hypothesis comes from an epidemiologic study of migrant Jews in Israel. Among Jews born in North Africa or Europe, the age-adjusted incidence rates of ADL increased
2-fold from 1962 to 1982. The rates did not increase at all among those born in Turkey (33). This finding may reflect the less urbanized, less industrialized environment and lower number of automobiles in Turkey.
Limitation
Our study is an ecological study and therefore has a major limitation: the data of exposure and disease obtained from populations cannot be linked to individuals. It caused a failure to control confounding factors like smoking. However, unlike most ecological studies, we used a three-dimensional analysis of time-place-population in this study. If we calculate a correlation coefficient from only one dimension, like most ecological studies do, it is possible that many factors can show similar correlation. It is less likely that one factor will show the same relation in time, place, and population dimensions simultaneously. Moreover, the results in this study are consistent with some case-control and cohort studies. This result is consistent with the result of a case-control study conducted by Nyberg et al. (34). They found that for an induction time of 21 to 30 years in those with the highest exposure to NO2, there is an estimated 44% increase in risk of developing lung cancer (relative risk, 1.44; 95% confidence interval, 1.05-1.99) compared with those who experienced the lowest levels, adjusting for age, year, smoking habits, radon exposure, and occupational exposures known to be associated with lung cancer. In addition, our result is consistent with the results reported by a prospective study and the case control study mentioned in Introduction. Vineis et al. (7) conducted a nested case-control study in 10 European countries and found a 30% increase in the risk of developing lung cancer (relative risk, 1.30; 95% confidence interval, 1.02-1.66) for those who were exposed to NO2 at levels >30 µg/m3 compared with those who were exposed to NO2 at levels <30 µg/m3. Pope III et al. (6), based on a prospective mortality study involving 1.2 million adults, reported that each 10-µg/m3 elevation in fine particulate air pollution was associated with an
8% increased risk of lung cancer mortality.
It is not clear whether NO2 is only an indicator of some real causes or it is the major cause. Further studies are warranted.
| Conclusion |
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| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Note: This study was supported by Mercer University, School of Medicine.
Ethics Committee Approval: No human or animal subjects were involved in the study.
Received 5/21/07; revised 9/20/07; accepted 9/28/07.
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