HEPA FILTERS & F8 only to be used in modified ventilation systems

MERV 17-20 Air Filters: Also known as HEPA air filters (High Energy Particulate Air). These filters surpass the standard MERV rating by removing 99.97% of airborne particles 0.3 micron and larger (F8 filter). While often used in hospitals, they cause too much air resistance to be installed in a standard HVAC or ventilation system without making modifications first so can be a problem for the fan motors found in most ventilation systems in NZ. 

In short these filters should not be used unless they are in a system with a fan designed for that type of filtration, F8's have been introduced only recently into systems that were designed for F6 & F7 filters.