2.8. Ethical Considerations 
HHS
The HHS Study adheres to all Danish ethical standards. The Regional Scientific Ethical Committee, the Capital Region of Denmark, reviewed the HHS Study and concluded that formal ethical approval was not required (J.nr. 16018722). The trial is registered and listed in the Danish Data Protection Agency (J.nr. 2015-57-0008). Written information was sent to principals, teachers, and student councils at all high schools which were invited explaining the implications of participating in the study. The participants were informed that their participation was voluntary, that their information would be used for research purposes only, and that their data would be treated confidentially. By participating, they gave consent that their data could be used for research.