AIRBORNE FINE PARTICLES
Besides gases and vapors, the air carries finely divided particulate matter. Particles of different sizes have different health effects.
Among the most common categorizations imposed on particulates are those with respect to size, referred to as fractions. As particles are often non-spherical (for example, asbestos fibers), there are many definitions of particle size. The most widely used definition is the aerodynamic diameter. A particle with an aerodynamic diameter of 10 micrometers moves in a gas like a sphere of unit density (1 gram per cubic centimeter) with a diameter of 10 micrometers. PM diameters range from less than 10 nanometers to more than 10 micrometers. These dimensions represent the continuum from a few molecules up to the size where particles can no longer be carried by a gas.
The notation PM10 is used to describe particles of 10 micrometers or less and PM2.5 represents particles less than 2.5 micrometers in aerodynamic diameter.[3]
But because no sampler is perfect in the sense that no particle larger than its cutoff diameter passes the inlet, all reference methods allow a high margin of error. These are also sometimes referred to with other equivalent numeric values. Everything below 100 nm, down to the size of individual molecules is classified as ultrafine particles (UFP or UP).[4]

Note that PM10-PM2.5 is the difference of PM10 and PM2.5, so that it only includes the coarse fraction of PM10.

AIR PURIFICATION
An air purifier is a device which removes contaminants from the air. These devices are commonly marketed as being beneficial to allergy sufferers and asthmatics, and at reducing or eliminating second-hand tobacco smoke. Commercial grade air purifiers are manufactured as either small stand-alone units or larger units that can be affixed to an air handler unit (AHU) or to an HVAC unit found in the medical, industrial, and commercial industries.
Dust, pollen, pet dander, mold spores, and dust mite feces can act as allergens, triggering allergies in sensitive people. Smoke particles and volatile organic compounds (VOCs) can pose a risk to health. Exposure to various components such as VOCs increases the likelihood of experiencing symptoms of sick building syndrome. Additionally, with the advancement in technology, air purifiers are becoming increasingly capable of capturing a greater number of bacterial, virus, and DNA damaging particulates. Air purifiers are used to reduce the concentration of these airborne contaminants and besides being very useful for people who suffer from allergies and asthma, recent technological and scientific studies are finding that poor air quality is also a contributing factor of some forms of cancer, respiratory illnesses, COPD, and other pulmonary infections and illnesses.
Air purifiers also reduce the need for frequent room and area cleaning.

HOW FILTERS WORK
Filter - based purification traps airborne particles by size exclusion. Air is forced through a filter and particles are physically captured by the filter. There are several different types of filters.
Inertial separators are used to eliminate particles outside of a certain size range. If a gas stream containing particles of different sizes is forced to turn a sharp corner, the inertia of the large particles causes them to separate from the gas stream lines. The larger particles can be collected and removed from the gas stream after collisions with the walls of the vessel. The two common types of inertial separators are cyclones, which spin the gas stream, causing collisions of the heavier particles with the outside of the cyclone wall in larger types of equipment, and impactors, where the gas particle stream is directed at a greased metal plate and turned at the last moment, causing the larger particles to stick to the greased plate.
Besides there are also several other different types of filters that capture particles down to a certain size.

WHAT IS A HEPA FILTER?
High Efficiency Particulate Air, or HEPA filters are designed to capture particles down to a size of 0.3 micrometres, but some bacteria and most viruses are much smaller.
HEPA filters remove at least 99.97% of 0.3-micrometer particles, and are usually more effective for particles which are larger or slightly smaller. They are effective down to 0.01 micrometers in many cases, but become ineffective for particles smaller than 0.01 micrometer. A HEPA filter may follow an easily cleaned conventional filter (prefilter) which removes coarser impurities so that the HEPA filter needs cleaning or replacing less frequently. HEPA filters do not generate ozone or harmful byproducts.

CARBON FILTERS
Activated carbon is a porous material that can adsorb volatile chemicals on a molecular basis, but does not remove larger particles. Activated carbon is normally used in conjunction with other filter technology, especially with HEPA.

ELECTROSTATIC FILTERS
Ionizer purifiers use charged electrical surfaces or needles to generate electrically charged air or gas ions. These Ions attach to airborne particles which are then electrostatically attracted to a charged collector plate. This mechanism produces trace amounts of ozone and other oxidants as by-products. Most ionizers produce less than 0.05 ppm of ozone, an industrial safety standard. There are two major subdivisions: the fanless ionizer and fan-based ionizer. Fanless ionizers are noiseless and use little power, but are less efficient at air purification. Fan-based ionizers clean and distribute air much faster. Permanently mounted home and industrial ionizer purifiers are called electrostatic precipitators

AIRBORNE MICROBES AND VIRUSES
HEPA filters are designed to capture particles down to a size of 0.3 micrometres, but some bacteria and most viruses are much smaller.

ULTRAVIOLET GERMICIDAL IRRADIATION
Ultraviolet germicidal irradiation - UVGI can be used to sterilize air that passes UV lamps via forced air. Air purification UVGI systems can be freestanding units with shielded UV lamps that use a fan to force air past the UV light. Other systems are installed in forced air systems so that the circulation for the premises moves airborne micro-organisms past the lamps. A good filtration system will remove the dead micro-organisms. A UV lamp placed at the coils and drainpan of cooling system will keep micro-organisms from forming in these naturally damp places.
Small equipment will sometimes include UV lamps to add germicidal irradiation to other types of air purification.

AIR POLLUTION AND OZONE
Ozone generators produce ozone (O3), and are sometimes sold as whole house air cleaners. Unlike ionizers, ozone generators are designed to produce significant amounts of ozone, a strong oxidant gas which can oxidize many other chemicals and also airborne microbes and viruses. The only safe use of ozone generators is in unoccupied rooms.
The use of "shock treatment" commercial ozone generators that produce over 3000 mg of ozone per hour is reserved for cleanup operations. Restoration contractors use these types of ozone generators to remove smoke odors after fire damage, musty smells after flooding, mold (including toxic molds), and the stench caused by decaying flesh which cannot be removed by bleach or anything else except for ozone. However, it is not healthy to breathe ozone gas, and one should use extreme caution when buying a room air purifier that also produces ozone
As with other health-related appliances, there is controversy surrounding the claims of certain companies, specifically involving ionic air purifiers. Particularly, some ionic air purifiers generate ozone, an energetic allotrope of three oxygen atoms, and in the presence of humidity, small amounts of NOx. Ironically, people who have asthma and allergy are most prone to the adverse effects of high levels of ozone For example, increasing ozone concentrations to unsafe levels can increase the risk of asthma attacks. Due to the below average performance and potential health risks, Consumer Reports has advised against using ozone producing air purifiers. Ozone generators used for shock treatments (unoccupied rooms) which are needed by smoke, mold, and odor remediation contractors as well as crime scene cleanup companies to oxidize and permanently remove smoke, mold, and odor damage are however considered a valuable and effective tool when used correctly for commercial and industrial purposes. However, there is a growing body of evidence that these machines can produce undesirable by-products as evidenced by an Environmental Protection Agency report that can be found at http://www.epa.gov/iaq/pubs/ozonegen.html
In September 2007, the California Air Resources Board announced a ban of in-home ozone producing air purifiers. This law, which took effect in 2009, will require testing and certification of all types of air purifiers to verify that they do not generate excessive ozone. This ban does not affect shock treatment ozone generators however for commercial and industrial use. Studies indicate that some ionic air purifiers produce ozone at 3.3 to 4.3 mg. Ozone generators used for shock treatments on the other hand produce over 3000 mg/h, the amount of ozone needed to create a "shock treatment" over a 6 hour period in a 100-200 sq ft room. The ozone generators which are subject to this ban use ceramic Mica plates that produce 300-400 mg/h of ozone and are intended to be used indoors in occupied rooms.
The use of these smaller ozone generators to eliminate mold, other fungus, algae and bad odors in basements and other UNOCCUPIED areas is allowed in all other states. Another example is bad odor due to seepage from a broken outside sewer pipe into a basement through its walls.

IONIC AIR PURIFIERS
Air ionizers are used in air purifiers. Airborne particles are attracted to the electrode in an effect similar to static electricity. These ions are de-ionized by seeking earthed conductors, such as walls and ceilings. To increase the efficiency of this process, some commercial products provide such surfaces within the device. The frequency of nosocomial infections in British hospitals prompted the National Health Service (NHS) to research the effectiveness of anions for air purification. Recent SARS outbreaks have fueled the desire for personal ionizers in the Far East, including Japan (where many products have been specialized to contain negative ion generators, including toothbrushes, refrigerators, air conditioners, air cleaners and washing machines). There are no specific standards for these devices. Notebook producers ASUS have now started to include air ionizers in their computers[2].

IONS VERSUS OZONE
Ionisers should not be confused with ozone generators, even though both devices operate in a similar way. Ionisers use electrostatically charged plates to produce positively or negatively charged gas ions that particulate matter sticks to (in an effect similar to static electricity). Ozone (O3) generators are optimised to attract an extra oxygen ion to an O2 molecule, using either a corona discharge tube or UV light. Even the best ionisers will produce a small amount of ozone, and ozone generators will produce gaseous ions of molecules other than ozone (unless fed by pure oxygen, not air).
At higher concentrations, ozone can also be toxic to air-borne bacteria and viruses, and may destroy or kill these sometimes infectious organisms. However, the needed concentrations are toxic enough to humans and animals that the FDA explicitly demands ozone therapy not be used as medical treatment, and has taken action against businesses that fail to comply with this regulation. Pure ozone is a highly toxic and extremely reactive gas. A higher daily average than 0.1 ppm (0.2 mg/m³) is not recommended and can damage the lungs and olfactory bulb cells directly.

HUMIDITY IN THE AIR YOU BREATHE
Our lungs require the air we breathe to contain a certain amount of water vapor. In the winter, our houses and buildings are heated and the relative humidity in the inside air is reduced, that is, the air becomes very dry. Note that outside in the open the humidity is also very low in cold weather because the cold has forced the precipitation of the airborne humidity as rain or snow. Many people will start coughing and feeling their dry throats when the air is dry. In order to correct this low humidity, we have to add water to the air we breathe indoors, using humidifiers.
In the summer, when the outdoors humidity raises with the heat, the air inside our houses and buildings will be very humid too. Air conditioning will reduce that humidity, together with the temperature. In the absence of air conditioning, a dehumidifier will lower the ambient humidity in the air. This also applies to moist cellars, basements and some ground floors too, where humidity seeps in through the walls from outside wet soil or from a high water table. Reducing the humidity with a dehumidifier will fight the growth of molds, fungus and algae which, besides being a eyesore and smelly, can trigger allergies due to their toxic spores. To fight molds, fungus and algae, other than periodically washing the walls and floors with bleach, you can install an ozone generator in areas where there are no people, or when people are not present (prohibited in the state of California).

DIFFERENT TYPES OF HUMIDIFIERS
There are basically 3 main types of air humidifiers:
Hot Steam humidifiers heat and boil a small amount of water in a well and the steam raises naturally through the air. The outside of the equipment remains cold and only the steam exits through a chimney. Inside the well will accumulate a crust made of the minerals dissolved in the water, and their quantity will vary with the region where they are used. The well have to be cleaned periodically to remove this crust (easier to do when wet, rather than dry) and the periodicity will depend on the amount of minerals in the water in the region where you live.
Evaporative humidifiers move some water into elements made of felt, fibers or sponge materials and a small ventilator makes the air flow through or around those wet elements promoting the natural evaporation of the water. Algae and fungus tend to grow and accumulate on those wet materials and they will have to be replaced periodically. Some manufacturers sell anti-fungal, anti-algae chemicals to be added to the water to avoid or reduce the growth of those organisms. The elements will still have to be replaced periodically, but will last longer.
Aerosol humidifiers blow water particles into the air from a nozzle, in the form of an aerosol consisting of a fine mixture of water and air. These humidifiers are often small and less expensive than the others but, as the water particles dry in the air, they will leave the minerals contained in the water as a fine airborne dust, which you may not want to breathe.

HOW DO DEHUMIDIFIERS WORK?
Dehumidifiers are basically small air conditioners that cool and condense the humidity from the air, removing it to a tank or to a pipe that will let it flow to a drain. When the tank is full, the machine will automatically stop working, until you empty the tank (about once a day).

HEATERS
These are well known equipments and don't require much explanation. The heat is usually produced by an electric resistance or coil and transmitted directly to an airstream flowing through it, or else to an oil tank, to stone plates or to metal fins, from which it flows into the surrounding air by convection. A ventilator is usually added to increase the air flow around the hot elements. You will usually have a choice of 2 or more levels of heat. Some heaters work at lower temperatures to increase safety to children and pets. Some will shut off if tilted or toppled. Most have thermostats that shut them off when a certain predetermined temperature is attained, and turn them on again when the environment cools off below a certain temperature. The name Radiators comes from the transmission of some of the heat directly through the air as infra-red rays – when you are close to a radiator, especially a glowing heat coil, that is the heat you feel on your exposed skin. The remaining heat flows into the surrounding air by convection.