Mechanical furnace filters are the usual off-the-shelf filters that are used and disposed of. They draw air in and trap particles. Some are washable / reusable and, with regular cleaning, can last one to five years. They are usually much cheaper than gas phase orelectronic furnace filters, but most don't work as well.
Mechanical furnace filters come in a variety of sizes:
Factors increasing MERV ratings
Sometimes called panel filters, flat furnace filters can be composed of mats of glass or cellulose fiber, wood fill, animal hair, synthetic fibers or even slit-and-expanded aluminum. A common flat-filter material is fiberglass. Flat filters typically remove an insignificant number of small particles and are mainly used to protect the furnace components from damage caused by large dust particles.
Pleated filters provide a big increase in filtering capacity by increasing the surface area of the filter. When stretched out, they are several times longer than a regular filter, and when in their compact form, they can catch and hold more particles than a flat model. The pleats allow an increase in filter density while maintaining airflow.
High Efficiency Particulate Air (HEPA) filters
HEPA furnace filters have an efficiency rating of at least 97.97 percent for all particles of 0.3-micron diameter. They are the top of the line in mechanical air filters but are rarely used in residential HVAC systems because of their size, price (up to $100) and high electricity requirements. Beware of less-efficient products billed as "HEPA-type" filters - their effectiveness at capturing particles and smoke can be much lower.