Ambient temperature, solar radiation and sickness absence due to mental disorders in Finland: A distributed lag non-linear regional analysis

Background Changes in ambient temperature and solar radiation have been associated with mental health but their relationship with sickness absence due to mental disorders is unclear. Methods We linked nationwide weekly sickness absence registers with meteorological data (2006 to 2017), collected from all 22 mainland counties and 5 larger areas in Finland, and used distributed lag non-linear models (DLNM) to explore potential non-linear and delayed relationships. Results Our results indicate higher levels of sickness absence when solar radiation is low. We did not find statistically significant associations at cold temperature extremes even though the findings pointed towards higher sickness absence levels at colder extreme temperatures. The modelling showed lower levels of sickness absence when the temperature was more than + 10 °C. For solar radiation, sickness absence rates were higher when solar radiation was less than approximately 8000 to 9000 KJ/m² and lower when it exceeded 10,000 to 12,000 KJ/m². To some extent, the relationships of sickness absence with both temperature and solar radiation reflected the climatic differences between the northern and southern parts of the country. Conclusions In Finland, higher exposure to temperature and solar radiation are associated with lower levels of sickness absence due to mental disorders whereas colder temperatures and lower amount of solar radiation are associated with higher levels. The associations for low solar radiation were more robust than those for low temperatures, with a typical delay of up to a couple of weeks.