In this cross-sectional study, we investigated the prevalence of oral mucosal changes in the Northern Finland Birth cohort (1966) of 1961 participants at the age of 46 years. The overall prevalence of any mucosal changes was 10.5%, of which 4% were grouped as potentially malignant disorders: OLD was found in 3.5% and leukoplakia in 0.5% of the study population. Unlike alcohol intake, both current smoking and snuff use significantly increased the number of mucosal changes. In general, males had mucosal lesions more often than females, related to their drinking, smoking and snuff use habits.
The final diagnoses collected were based on documentation by a general dentist examiner that was later simultaneously re-evaluated by two oral mucosal specialists. The agreement between the general dentists and specialist diagnoses was low (54.8%) compared to a Swedish multicentre dental patient study, where 85% of the 803 cases were given the same diagnosis by the general practitioner and two oral mucosal specialists . In our cohort, 20.9% of the mucosal changes documented were not given any primary diagnosis by the general dentists and were only classified by the specialists. This may be because the general dentist examiners were unfortunately not well trained in the diagnosis of oral mucosa but were better trained in classifying teeth and gingival diseases, which were examined during the same visit. The lack of biopsy may also hinder with our diagnoses, which is a point that should be taken into account in further investigations.
The prevalence of oral mucosal changes varies depending on the published study populations [1,2,3,4,5,6, 10, 17,18,19,20,21,22,23,24,25,26,27,28]. For example, in a recent evaluation of Slovenian citizens (2395 patients, aged 22–92 years) who attended a general dental practice, the prevalence of OMLs was 27% . A similar result was found in an Italian study (4098 subjects, age range 19–96 years) where the prevalence of OMLs was 25.09% . However, in a southern Indian study (1190 patients, age range 2–80 years) conducted in a specialist clinic, the prevalence of one or more OMLs was clearly higher, 41.2% . In a large Chinese general population (n = 11,054) of 1- to 96-year-old inhabitants , the prevalence of OMLs (10.8%) was almost the same as in our cohort (10.5%), but less than in a Swedish study (6448 adults, mean age = 56.0), where OMLs were found in 14.7% of the participants . The higher prevalence in the Slovenian, Italian, Indian and Swedish cohorts may also reflect the heavy use of tobacco in Slovenia and Italy, betel in India, and snuff in Sweden, which increase the incidence of OMLs.
Here, the ten most common mucosal findings were OLR, OLP, Fordyce granules, hyperkeratosis, fissured tongue, geographic tongue, fibroma, snuff-related lesions, amalgam pigmentation and median rhomboid glossitis. When comparing our results to other publications, similar lesions are often recorded, but their prevalence seems to vary depending on the study population [1,2,3]. Some studies have focused on analysing mucosal lesions that raise a risk of oral cancer, or are associated with risk habits, such as smoking or denture wearing [22, 23, 25, 27]. In our study, we did not compare the presence of OMLs to chronic irritation, like dentures, which would have been worth conducting. The population reports have often focused on the prevalence of OMLs in adults of all ages including sometimes children sometimes not [1, 3, 17, 19, 27], therefore, it is not possible to directly compare results of our cohort of 45–47-year-olds with those.
Of normal variations, we detected mostly Fordyce granules, in 1.2% of the participants, whereas in the Chinese all-aged general population, the prevalence was only 0.5% , and in the Swedish adult study Fordyce granules were not listed . generally, the prevalence of Fordyce granules has been higher than in our cohort: in Slovenia 1.9% , in Turkey 2.8%  and in India 6.6%  of the study population.
The most common OML was a white lesion group (6.5%) including OLP, OLR, hyperkeratosis, snuff-related lesion, leukoplakia and candidiasis, found more often in males (8.1% vs. 5.1%). In a Turkish study , white lesions were found in only 2.2% of the 5000 consecutive 17–85-year-old patients, whereas ulcerated lesions were recorded most often (6.6%), which we recorded in only 0.2% of participants.
The prevalence of OLP was 1.5%, similar as in Italy (1.46%), but higher than in China (0.8%), Turkey (0.8%), India (1.3%), or a more recent study from Slovenia (1.1%) [1, 3, 19, 21, 25]. However, in an earlier Slovenian study, OLP was found twice as often as the more recent study, in 2.3% of the participants . In Cambodia, 1.8% of 1319 individuals studied had OLP . Overall, our result of the OLP prevalence seems to fit well within the various reports ranging from 0.8% to 2.3%. Of the OLP patients, 62% were females, and it affected 1.8% of all females and 1.2% of all males. In a Swedish study (of over 30-year-olds), OLP was also more frequent among females (2.2% vs. 1.6%) , and in Slovenia, OLP was up to twice as common in females (3% vs. 1.5%) . Interestingly, in a Cambodian population, OLP was only detected in females , whereas there were no sex-related differences in the prevalence of OLP in a Turkish study . These results indicate that OLP in most, but not all, populations seem to be more common among females.
When we compared OLP with OLR, the prevalence of OLD was 3.5%, which is higher than in the most recent Swedish study , as well as in the Italian study (2.4% and 1.75%, respectively). Our higher prevalence may also be due to some misdiagnoses based on clinical images only, without biopsies, which were taken in the Italian study to confirm the diagnoses .
We registered leukoplakia in 9 (0.5%) of the participants, of which 5 were current, 2 former and 2 non-smokers. The prevalence of leukoplakia in our study was in a similar range as in recent Swedish  and Slovenian  studies where leukoplakia was found in 0.4% and 0.5%, respectively. However, in the Swedish study from 30 years ago, the prevalence of leukoplakia was higher (3.6%), and most of them (2.9%) were associated with tobacco . In a recent Indian study, leukoplakia was detected in 5.7% of the 300 participants . There are two studies from Italy in which the prevalence of oral leukoplakia varies significantly; in the randomly selected male participants over 40 years of age (n = 118) from northern Italy , the prevalence of leukoplakia was 13.8%, whereas of the 4098 male and female participants in the Turin area, leukoplakia was registered in 1.15%. In both studies, leukoplakia was more common in current smokers than in never smokers [10, 28]. Since smoking is more popular in India and Italy than in Finland currently, that can explain the higher number of leukoplakia lesions in those populations.
Smoking in Finland has decreased during the last decades, and in 2018, of the 20–64-year-old population, 14% were daily smokers , but in this northern Finland cohort 18% were still active smokers. The three most common lesions in smokers were OLP, hyperkeratosis and OLR. Current smokers had also more OLP than former or non-smokers, and current female smokers had OLP more often than males (5.7% vs. 1.8%). In Slovenia, OLP was diagnosed in only 1.5% of smokers (n = 392) , whereas in our study OLP was more than twice as common in current smokers (3.8%). Interestingly, for some reason geographic tongue was significantly more common in non-smokers (1.5%) than in current smokers (0.6%), similar to that seen in a Swedish study where 6.8% of non-smokers and 1.7% of smokers had a geographic tongue .
The use of snuff is lower in Finland compared to Sweden, but it has recently increased, especially among the youth. Although snuff is not sold in Finland, Swedish snuff is brought to Finland from Sweden. Based on the Finnish National Institute for Health and Welfare statistics 2018, about 5% of Finnish 20–64-year-old males and 0–1% of females used snuff daily . In our cohort, the number of snuff users was 3.7% (n = 70), 44% used it regularly, and 2 were females. The three most common lesions in snuff users were snuff-related local lesions (14.3%), Fordyce granules (5.7%) and OLR (2.9%). None of the snuff users had OLP or leukoplakia. In the most recent Swedish report, snuff dipper’s lesion was found in 4.8% of the study population, but in that study the snuff users other mucosal changes were not recorded .