The reason the instructions say to use it horizontally is a government (EPA) standard. They wont allow Thermacell to list other means of use. They (EPA) don't want Thermacell to advertise it for use while attached to yourself or any other way because of a pressurized butane cartridge and the fire risk. Thermacell has NEVER had an issue with the cartridges or any fire issues. You can clip it to your pack or belt and as long as you aren't moving too fast (running or riding an ATV) it will work just fine.