Our study confirms a strong association between low SES and risk of OSCC in eastern Golestan. Several indicators of SES, including formal education, being married and high-wealth status were inversely associated with risk of OSCC. Education level showed a strong dose–response inverse association that was unaffected by adjustment for several potential confounders. Marital status also showed a strong inverse association that was largely unaffected by adjustment. The protective effect of high wealth score, however, was only seen for the highest quintile and disappeared after adjustment for other variables (mainly vegetable intake).
Using education as a marker of SES has several advantages and disadvantages. Questioning about education usually produces reliable results and is not affected by recall bias. Another advantage is that it is not altered by health status in old age. Education level is unlikely to change after early adulthood and it has been shown to be the main indicator of childhood SES with respect to some future health outcomes, such as overall mortality, as reported in a major review of prospective studies.29
One disadvantage of using education as a marker of SES in our study population was the usually limited years of schooling among most older people.15
The inverse association between education level and risk of OSCC found in our study is in agreement with the previous literature.7,30
While this association is strong and consistent across studies, and is therefore most probably real, it is difficult to know exactly how education affects risk of OSCC. Higher education may reflect a higher SES status of the family during childhood, which may have an effect on future health. People with higher levels of education may get jobs with higher income.15
In addition to years of schooling, early childhood education itself seems to have health benefits.14
One of the non-economic social effects of education is acquiring general and health-related knowledge, which can affect health outcomes.17
In our study, widowed/divorced (mainly widowed) people were at a higher risk of OSCC. This is consistent with a few other studies that have reported an association between being widowed or divorced and higher mortality from oesophageal cancer, or an inverse association between duration of living with a partner and risk of OSCC.31,32
We can only speculate on the reasons for these observations. Being widowed or divorced would result in loss of emotional support by the former spouse, and could result in less income, fewer social relationships, and less balanced nutrition. Nonetheless, adjustments for vegetable intake did not change our results.
We evaluated several variables related to living in more or less crowded environments, including number of siblings (related to one's environment during childhood), number of children, number of people living in the household, total house area and house area per person living in the household as potential indicators of SES. People with larger families and more crowded accommodations may have less adequate diets and higher exposure to infections.33,34
On the other hand, in more traditional societies with closer family relationships, people with larger families may be better supported by family members and may have closer social relationships.35,36
In our study, the number of siblings, the number of children and the number of people living in the household did not have independent associations with risk of OSCC, even though these variables and the wealth score had high loading factors in the second factor of our factor analysis (), which indicates that they were correlated. The house area per household member showed a positive association with risk of OSCC, which probably reflects the importance of living closely with family members in this society.
Collecting data on income and savings can be a sensitive issue in epidemiological studies.15,17
In addition, income may not reflect the true economic status of some individuals, especially older people, who usually do not work.15
Therefore, other measures of wealth may be more useful economic status indicators for these people.18
Collecting information on non-monetary indicators of wealth, such as owning appliances, can be relatively simple and a less sensitive issue,37
particularly in non-Western countries.38
In studies conducted in Golestan in the 1970s, only the number of cows owned at the age of 25 years showed an inverse association with risk of oesophageal cancer; other indicators of cattle or farm ownership were not associated with the risk.7
We asked about ownership of household appliances as a measure of wealth. Owning an automobile, which is more expensive than most other appliances and could be considered as an indicator of high income, was inversely associated with risk of OSCC. The MCA-based wealth score and owning an automobile were correlated (Spearman's rank correlation coefficient = 0.53; Appendix Table 1 available as supplementary data
online). However, the protective association of owning a car persisted after adjustment for the wealth score, so owning a car may also be a proxy for some other more direct factors. For example, it may provide convenient access to food shopping or it may enhance social contacts.15
The protective effect of high wealth scores was observed only in the group with the highest wealth status; interestingly, there was no difference in the association between wealth score and risk of OSCC among the least wealthy 80% of the population.
In developed countries, occupation can be a good surrogate for income and social prestige, and it may also correlate with education level. Nonetheless, income inequalities can exist within one occupation, e.g. between ethnic groups or between men and women.15,39
In addition, it is not clear whether the husband's occupation is a good SES indicator for a housewife.15
In our study, there was limited variation in the occupation types; >25 and 65% of study subjects in urban and rural areas, respectively, were farmers. Although some occupations were associated with higher wealth score, none of the occupation categories was associated with risk of OSCC.
Duration of residence in a rural area showed a positive association with risk of OSCC, despite the fact that this association might have been attenuated due to matching cases and controls on current place of residence. A recent study has shown that the rural dwellers in Golestan have much lower vitamin intake than the urban dwellers,40
which might explain some of the increased risk of OSCC in this subgroup.
If improved SES is indeed an important factor in the observed recent decrease in OSCC incidence in Golestan, one might expect that further improvements in SES could lead to a further reduction in the incidence of this disease. Changing some factors that influence SES, such as the general economy, may not be achievable in the short term, and it may be beyond the reach of public health programs. However, if SES influences health outcomes via its effect on behaviour and lifestyle, one practical way to reduce OSCC incidence may be to conduct active educational programmes to increase general awareness of known and potentially important risk factors for this disease. Known risk factors of OSCC in Golestan include low fruit and vegetable intake and tobacco or opium consumption6,7
and other suggested risk factors may be poor oral hygiene,41
drinking hot tea,8
and high exposure to some environmental factors like polycyclic aromatic hydrocarbons42,43
The protective effect observed in our study for attending even primary school, which can help children acquire health-related knowledge, suggests the potential importance of increasing disease-related awareness in the general population.
This study has several strengths and limitations. The strengths include histological proof of OSCC, administration of pre-tested structured questionnaires by well-trained interviewers, and adjustments for several potential confounders. In addition, we utilized different methods to investigate associations between SES indicators and risk of OSCC, which all showed consistent results. One of the limitations of this study is the retrospective assessment of exposures. However, we mainly asked questions about easily recalled facts, such as educational level or ownership of residence or appliances, which reduces the possibility of recall bias. Although recruiting neighbourhood controls can attenuate some associations, it provides efficient control for some factors and thereby reduces the possibility of spurious associations. By selecting controls randomly from an updated family health census and having a good response rate, the possibility of selection bias was minimized.
In conclusion, the results of this study confirm the inverse association between SES and OSCC risk seen in most other studies,30
and they particularly confirm the results of the one previous case–control study conducted in Golestan. The presence of an SES effect on risk of OSCC after adjustment for known more direct risk factors may indicate that there are some yet unidentified risk factors for OSCC which are correlated with SES measures. Further studies are needed to identify these risk factors. The results of this study also suggest that conducting an active program to increase general awareness of known and probable OSCC risk factors may be helpful in further reducing the incidence of OSCC in Golestan. Finally, our results also demonstrate the importance of using multiple SES indicators in epidemiological studies.