Air freshener

Mechanisms for the control of indoor airborne odors can be grouped into various different classes:

• Masking: a pleasant odor may be used to overwhelm unwanted odors.
• Adsorption: adsorbents like zeolite, activated charcoal,[11] or silica gel may be used to capture odor-producing compounds.
• Oxidation: ozone can oxidize and eliminate organic sources of odors from the air.
• Disinfection: odors caused by microorganisms can be removed by inactivating them using air sanitizers.

Delivery of the above air freshener mechanisms falls into two broad categories: continuous action and instant action.

Continuous action products include scented candles and devices which use a candle flame or some other heat source to heat and vaporize a fragrance formulation, incense burners, wall plug-ins which either use piezoelectric technology to aerosolize fragrance or heat to vaporize it, fragrance impregnated gels which release fragrance as the gel evaporates sometimes with the help of an electric fan, wick and reed diffusers which release fragrance by evaporation from fragrance-soaked wicks or wooden reeds; and fragrance impregnated materials like floor wax, paper, plastics, wood which release fragrance by off gassing; and lastly nebulization systems which convert liquid fragrances into a vapor in a cold process without the use of heat.

Instant action systems are mainly aerosol or atomizer sprays. The aerosol spray uses a propellant and fragrance packaged under pressure in a sealed metal container with a valve which is opened by pressing down a button which contains a spray nozzle – the actuator. When the container's valve is opened by pressing the actuator, fragrance is forced through the spray nozzle located inside the actuator to create a mist of droplets containing fragrance. These droplets are 30 to 50 micrometres in diameter. A recently developed alternative, the "bag-on-valve" aerosol, places the air freshener inside a bag in the can. The can is filled with pressurised air which squeezes the bag and pushes out the product when the actuator is pressed.[12] An atomizer operates in a similar fashion except that the actuator is a pump which when pressed a few times creates the pressure to aspirate the fragrance from the container through a tube into the actuator and spray nozzle. Because the container is not constantly under pressure it may be made of glass or plastic as well as metal. The mist created contains droplets 50 to 150 micrometres in diameter.

Air fresheners use both synthesized and organically occurring fragrances. Common fragrances used include lyral and citronellal. Preparations often include terpenes such as limonene.

A propellant is usually a liquid gas substance used to physically propel the product out of an aerosol spray. The substance chosen needs to be a gas that turns to liquid when pressurized with a relatively low amount of pressure, safe for domestic use. The propellant is a liquid when under pressure in the can, but maintains the pressure in the can (and thereby its spraying ability) by turning partially back to a gas to fill any empty space. Less common compressed gas propellants work similarly but maintain pressure by filling the empty space without liquefying. The propellant in a true aerosol is mixed with the air freshener, so it must also not react with the product. Sometimes, multiple propellants are combined.

CFCs were once popular propellants but are no longer widely used because of their damaging effects on the ozone layer and resulting bans. Current propellants used in air freshener sprays include:

• hydrocarbon aerosol propellant (HAP) blends: mixtures including propane, n-butane and isobutane[13]
• dimethyl ether[14]
• hydrofluorocarbon propellants (HFCs) such as 1,1-difluoroethane[15]
• hydrofluoroolefin propellants (HFOs) such as 2,3,3,3-tetrafluoropropene[16] HFOs have recently become more widely used due to their low vapor pressure, low global warming potential (GWP), and nonflammability.

Note that while bag-on-valve systems may use compressed air, nitrogen or carbon dioxide or other substances to squeeze the air freshener from the bag, these are not strictly ingredients because they are not mixed with the air freshener, and remain in the can.

Preservatives prevent the decomposition of the product or the growth of microorganisms in it. These can include:

• parabens such as methylparaben[17] and propylparaben[18]
• isothiazolinones such as methylisothiazoline[19] and benzisothiazolinone[20]
• formaldehyde releasers such as DMDM hydantoin[21]

These are substances that help other ingredients mix together, either by dissolving or emulsifying.

Solvents used in air fresheners include ethanol, mineral oil, or glycol ethers such as 2-butoxyethanol and trideceth-4.

Many air fresheners employ carcinogens, volatile organic compounds and known toxins such as phthalate esters in their formulas.[22] A Natural Resources Defense Council (NRDC) study of 13 common household air fresheners found that most of the surveyed products contain chemicals that can aggravate asthma and affect reproductive development. The NRDC called for more rigorous supervision of the manufacturers and their products, which are widely assumed to be safe:

The study assessed scented sprays, gels, and plug-in air fresheners. Independent lab testing confirmed the presence of phthalates, or hormone-disrupting chemicals that may pose a particular health risk to babies and young children, in 12 of the 14 products—including those marked 'all natural.' None of the products had these chemicals listed on their labels.[23]

On September 19, 2007, along with the Sierra Club, Alliance for Healthy Homes, and the National Center for Healthy Housing, the NRDC filed a petition with the U.S. Environmental Protection Agency and the Consumer Product Safety Commission to report the findings.[24]

Research at the University of Colorado at Boulder revealed the carcinogenic nature of paradichlorobenzene (PCDB) and naphthalene, present in some types of air fresheners. The pesticide substances were mainly used in mothballs but also in room sprays and toilet rim blocks.[25][26]

The University of Bristol's Avon Longitudinal Study of Parents and Children (ALSPAC) found that exposure to volatile organic compounds through frequent use of air fresheners and other aerosols in the home was found to correlate with increased earaches and diarrhea in infants, and with increased depression and headaches in their mothers.[27][28]

In 2008, Anne C. Steinemann of the University of Washington published a study of top-selling air fresheners and laundry products.[29][30] She found that all products tested gave off chemicals regulated as toxic or hazardous under federal laws, including carcinogens with no safe exposure level, but none of these chemicals were listed on any of the product labels or Material Safety Data Sheets. Chemicals included acetone, the active ingredient in paint thinner and nail-polish remover; chloromethane, a neurotoxicant and respiratory toxicant; and acetaldehyde and 1,4-dioxane, both carcinogens. A plug-in air freshener contained more than 20 different volatile organic compounds, with more than one-third classified as toxic or hazardous under federal laws. Even air fresheners called "organic," "green," or with "essential oils" emitted hazardous chemicals, including carcinogens.

A report issued in 2005 by the Bureau Européen des Unions de Consommateurs (BEUC) found that many air freshener products emit allergens and toxic air pollutants including benzene, formaldehyde, terpenes, styrene, phthalate esters, and toluene.[31][32]

In the United States, since 2020 air fresheners (as well as cleaning solutions and products used to clean cars) have been required to list any of their ingredients which are on California's list of 2,300 harmful chemicals, based on a California law passed in 2017.[33] A California study in 2006 found that the prominent products of the reaction of terpenes found in air fresheners with ozone included formaldehyde, hydroxyl radical, and secondary ultrafine particles.[34] It is not clear if manufacturers will need to list such chemicals which are not ingredients, but form during deployment and are thus able to affect human health.

In 2009, Stanley M. Caress of the University of West Georgia and Anne C. Steinemann of the University of Washington published results from two national epidemiological studies of health effects from exposure to air fresheners. They found that nearly 20 percent of the general population and 34 percent of asthmatics report headaches, breathing difficulties, or other health problems when exposed to air fresheners or deodorizers.[35]

Air freshener aerosols with certain propellants are susceptible to misuse as an inhalant. Air fresheners have also been used to cause aerosol burn. In rare cases aerosol burns are reported to have been caused by air freshener canisters exploding.[36]