This study provides new evidence on occupational risk variation of NPC in the Nordic countries. Norwegian, Finnish and Swedish fishermen had elevated SIRs of NPC in the present study which to our knowledge is a new finding. Salted fish is a confirmed risk factor for NPC and commonly used in Southern Asia (27), but we no evidence whether fishermen in the three Nordic countries consume more salted fish than the reference population. Fishermen might have been exposed to off-gas from fermented fish, especially in the old time where the storage tanks on fishing boards were emptied manually.
Cooks and stewards showed elevated SIRs in the present study. Previously the elevated risk for lung cancer among cooks has been related to indoor air pollution which may also be one link between cooks and NPC (28). In addition, wood combustion has been found to be a potential risk factor for NPC and a potential exposure agent for cooks (29).
In some previous studies frequent exposure to cotton dust has been associated with elevated risk of NPC (12, 21, 30). We did not find an elevated SIR among female textile workers, and there was a statistically significant decreased SIR for male textile workers. Thus, this study does not support a such evidence.
We found that female launderers had statistically significantly higher risk of NPC than the population on average. In drycleaning, tetrachloroethylene is the dominant solvent used in the world and in the Nordic countries (31).
The International Agency for Research on Cancer (IARC) classified formaldehyde and wood dust as occupational carcinogens in relation to NPC (32). In the present study, exposure to these agents is most likely among wood workersbut we did not find elevated SIRs among them (33). NPC incidence was significantly elevated among male waiters. Previously Nordic waiters have been related to elevated incidence of lung cancer, oral cancer and pharyngeal cancer (34). An increased risk of death from NPC was reported among waiters in China (35). One explanation may be that waiters have been exposed to passive and active tobacco smoke, which has also been associated with NPC (4, 36). Prior to smoking restrictions in bars and restaurants, waiters worked in a smoke-filled environment and had higher proportion of active smokers than other workers (34). Several studies have shown a modestly increased risk of NPC associated with tobacco smoking, and a meta-analysis indicated an approximately 60% increased risk for NPC among smokers (37, 38). In Finland and Norway, smoke-free legislation in restaurants has shown effective results in the decrease of passive smoking exposure (36, 39) and improving lung function  among waiters. Therefore it is expected that the excess incidence of NPC among restaurant staff will get smaller in the future.
Strengths of this study include large cohorts with nationwide population-based data, long follow-up period and high quality registry data (26). A limitation of the present study was the missing stratification of NPCs by histological subtype, and the lack of information on factors not directly related to occupation.