Using a HEPA filter in a furnace is not recommended, as the dense filtration material used in these filters requires too much air pressure for a furnace fan to handle. Traditional air filters with MERV ratings of 12-1 can approach HEPA filtration levels, but they are not as effective at removing viruses. The only time it is acceptable to use a HEPA filter in a home is if the system requires it. These filters are more common in commercial applications, medical facilities, and high-tech manufacturing facilities.
For residential applications, air purification systems are the best option. These systems usually include a hospital MERV 16 filter, germicidal lights, and carbon filters. If you need additional filtration, you can use a disposable pleated air filter and place a portable air purifier with a true HEPA filter in the room. Manufacturers often recommend filters with a MERV rating greater than 8 for pet dander removal.
However, recent innovations have made it possible to obtain high MERV rates (11-1) with a low pressure drop. If you notice that smoking air is finding its way around your clogged filter, it is time to install a new one. Nordic Pure and Honeywell's FPR 9 and Filtrete MPR 1500 filters are widely available in retail stores and offer good filtration levels. In general, filters with MERV ratings of 1-16 range from those that can catch large particles to those that can catch just about everything. Considering the threat posed by the spread of COVID-19 and other germs, upgrading your building's air filter to one with a high MERV rating is the best way to go.