The main types of filters are usually found in central heating and air conditioning systems, in stand-alone filtration systems and even in vacuum cleaners.

Typical blanket filters, with a maximum thickness of 5 centimetres, are very ineffective as they only remove larger particles that are harmless to health. They are used more to protect office equipment than occupants.

Electrostatic filters are advertised as “highly efficient” in removing allergens and dust from the air. This is just a passing trend. When clean, they are somewhat more efficient than blanket filters, but they do get dirty very quickly, losing a great deal of efficiency. Their main advantage is that they are washable and do not require new filters to be bought. The main drawback is that they have to be washed continuously. Advertisements that promote “95%” efficiency are misleading as this refers to the weight percentage of dust removed and this corresponds to the heavier particles. Electrostatic filters are technically referred to as “low-efficiency impact filters”.

Pleated filters are more efficient; they are of medium efficiency and remove a greater amount of smaller particles, which are the most harmful to health. These filters vary in thickness: 15 cm, 20 cm or 30 cm, with the latter being the most effective. The problem is that some systems do not have a high enough flow for the air to pass through the filters, or enough space for the filters to be placed inside.

HEPA (High-Efficiency Particulate Air) filters were originally developed for the nuclear industry. They remove 99.97% of particles (even very small ones) and require extra drive power as they produce a large pressure drop. Virtually no unwanted matter passes through these filters.

All the filters mentioned above remove airborne particles. The most common filter used to remove gases is the Activated Carbon filter. These filters remove some, but not all, gaseous air pollutants. Activated Carbon, Potassium Permanganate and Zeolite (CPZ) filters remove more gases. These filters must have enough CPZ in order to be efficient, depending on the volume of air that the system is capable of moving.

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