Aerosol: Uses, Types, Examples

Many of us have stared at the astonishing beauty of sunrises and sunsets at some point in life. People, who live closer to the polar regions, might also have marveled at the eye-catching twilight hues. Both the observations are the effect of a physical phenomenon called the scattering of light. But what causes the light to scatter in clear air? Although it may seem crystal clear, it is almost certain that we inhale tens of millions of solid particles and liquid droplets, known as aerosols, when we take a deep breath. The term aerosol (short for “aero-solutions”) encompasses many kinds of little bits of stuff that end up suspended in the air (or other gases). They can be solid or liquid and infinitesimally small or big enough to be seen with the naked eye. Examples of natural aerosols are fog, mist, dust, forest exudates, and geyser steam. Aerosol science covers a wide area as there are various phenomena where one can encounter the presence of aerosols. The classifications can be made based on the generation and removal of aerosols, technological application of aerosols, effects of aerosols on the environment and people, and other topics. Let’s discuss a few types of aerosols one can come across in their everyday life.

Types of Aerosols

There is a common misconception that the term “aerosol” refers to the solid phase of the compound, whereas in reality and by definition, it is the combination of the gas and solid phases.

Atmospheric Aerosols

Atmospheric aerosols are suspensions of liquid, solid, or mixed particles in the air with highly variable chemical composition and size distribution. When these particles are sufficiently large, we notice their presence as they scatter and absorb sunlight. Their scattering of sunlight results in haze and redden sunrises and sunsets. Aerosols can have a major impact on climate when they scatter light. Particles large in diameter scatter most of their incident solar radiation in the forward direction, and less back to space. The larger the particle size, the greater percentage of the light is scattered forward. Larger aerosol particles thus contribute less to tropospheric cooling. Based on their source, atmospheric aerosols are fundamentally categorized into two types:

Primary Aerosols

Primary aerosols are atmospheric particles that are emitted directly into the atmosphere. It comprises both organic and inorganic compounds with particles size larger than 0.1 micrometers in diameter. These aerosols have short atmospheric lifetimes, typically of a few days. They get lifted into the atmosphere by gusty winds, exploding volcanoes, or waft away from smokestacks or flames.

Examples

• Black carbon: Black carbon, often called soot, is the most dominant light-absorbing particulate matter aerosol present in the air. It is primarily emitted by several combustion processes such as in diesel engines and wildfires. These particles are particularly known to be deposited on snow and ice, causing the absorption of incoming solar radiation, and thereby, melting the snow and leading to reduced albedo, which in turn leads to reduced tropospheric warming. Black carbon may also have adverse impacts on human health; however, unlike persistent greenhouse gases such as carbon dioxide, black carbon is removed from the atmosphere within 1-2 weeks, so its impacts tend to be more regional rather than global.

• Biomass Burning: Biomass refers to the plant or animal materials that are used as fuel to produce electricity or heat. Examples of biomass or biofuel include wood, energy crops, and waste from forests, yards, or farms. Burning such materials produces a variety of particulate matter such as nitrogen oxides, carbon monoxide, sulfur dioxide, lead, mercury, and other hazardous air pollutants, which may have a great impact on both the climate and human health. In general, these particles contribute to cooling the troposphere via scattering the sunlight, or by becoming Cloud Condensation Nuclei (CCN) that causes the formation of rain or snow.Nonetheless, some of these particles absorb incoming solar radiation, thereby contributing to tropospheric warming.

• Mineral Dust: Mineral dust is an atmospheric aerosol that originates from the suspension of minerals constituting the soil. It is composed essentially of clay and silt particles, whose diameters vary between 0.1 to 20 micrometers (∼1/10 of hair). Larger particles, such as sand, are not considered aerosols because they are eventually removed from the atmosphere by gravitational settling. Mineral dust can absorb or scatter incoming solar radiation. However, the scattering is observed more often, which leads to the troposphere cooling as a net effect. Mineral dust is also believed to play an important role in many marines and terrestrial biogeochemical processes, as the trace metals present on the dust are important nutrients for the development of various ecosystems.

• Sea Spray: Sea spray refers to aerosol particles that are formed directly from the ocean, mostly by ejection into the atmosphere by bursting bubbles at the air-sea interface. It contains both organic and inorganic that form the most widely distributed natural aerosols known as Sea Salt Aerosols (SSA). They are characterized as scattering, highly hygroscopic, and having a wide range of particle sizes. Due to the hygroscopy, a sea salt particle can serve as a very efficient cloud condensation nuclei (CCN), altering cloud reflectivity, lifetime, and precipitation process. In general, they have a cooling effect on the troposphere.

• Volcanic Aerosols:  Volcanic eruption causes several types of particulate matter to reach the stratosphere. Most of the aerosols present in the stratosphere are sulfur-rich particles ejected from the volcanoes. They are also known by the name Stratospheric sulfur aerosols. These particles consist of a mixture of sulfuric acid and water. Volcanic aerosols are a mix of absorbing and scattering particles, with the absorbers usually being large enough to fall out quickly so that the smaller ones dominate.

Secondary Aerosols

Secondary aerosols form when different things floating in the atmosphere like organic compounds released by plants, liquid acid droplets, or other materials crash together, culminating in a chemical or physical reaction. Natural secondary aerosols originate in the atmosphere as a result of in situ gas to particle conversion of condensable vapors. Gas-to-particle (g-to-p) conversion in the atmosphere can be of two forms: either the growth of an existing aerosol or the nucleation of new particles. The amount of aerosols produced by g-to-p conversions is comparable to primary natural aerosols and exceeds primary anthropogenic aerosols. Secondary aerosols are primarily composed of three families of compounds: sulfates, nitrates, and organic compounds. Let’s discuss a few of its examples.

Examples

• Sulfate Aerosols: The term “sulfate aerosols” is used for a suspension of fine solid particles of sulfate or tiny droplets of a solution of sulfate or of sulfuric acid ({H}_{2}{SO}_{4}). They are produced by chemical reactions in the atmosphere involving gaseous precursors, except for sea-salt sulfate and gypsum dust particles. Sulfate aerosols nucleate homogeneously or form on existing particles from gaseous precursors. Natural sulfate aerosols are formed from ({SO}_{2} emitted by volcanoes into the atmosphere and from dimethyl sulfide (i.e., ({CH}_{3}{SCH}_{3}, or referred to as DMS) emitted by biogenic sources, especially marine plankton. Sulfate aerosols are common and the scattering properties outweigh the absorbing properties to produce a net cooling effect on the troposphere. However, other particles from volcanoes can also be cloud condensation nuclei, enhancing their cooling effect.

• Nitrate Aerosols (NOx):  Nitrate aerosols are formed due to the presence of ammonia and nitric acid in the atmosphere. Ammonia and nitric oxides, which is a precursor of nitric acid, have a long list of anthropogenic and natural sources including excreta from domestic and wild animals, synthetic fertilizers, oceans, biomass burning, crops, human populations and pets, soils, industrial processes, and fossil fuels. The formation of ammonium nitrate aerosol depends on the thermodynamic state of its precursor and depends strongly on the environmental conditions. Gaseous ammonia and nitric acid react in the atmosphere to form aerosol ammonium nitrate, {NH}_{4}{NO}_{3}. The concentration of nitrate aerosols is of great importance as nitrates are one of the main climate driving aerosols present in the atmosphere. Although the impact of nitrate aerosols is less than greenhouse gases, the steady increase in their concentration shows that it can be the largest air pollutant by the end of the 2030s.

• Secondary Organic Aerosols (SOA): SOA particles originate in the atmosphere through the mass transfer of low-pressure products given by the oxidation of organic gases to the aerosol phase. Carbonaceous particles emitted directly into the atmosphere derive mainly from biomass fires. Organic and carbonaceous aerosols are produced by g-to-p conversion of gases released from the biosphere and volatile compounds, such as crude oil leaking to the Earth’s surface. The organic gases are oxidized in the gas phase by species such as the hydroxyl radical (OH), ozone (O3), and nitrate radical (NO3), so that such oxidation products gradually accumulate. Some of these products have low volatilities and condense on the available particles to establish equilibrium between the gas and aerosol phases.

Organic Aerosols

Atmospheric aerosol particles are known to contain organic carbon material in variable amounts, depending on their location. In some parts of the world, organic compounds make up the majority of the total suspended particle mass. Due to the richness of organic chemistry, organic aerosols are both emitted as primary aerosol particles and formed as secondary aerosol particles from the condensation of organic gases. Although organic aerosols probably scatter more light than they absorb (cooling effect), they can also lead to the formation of ozone which is a greenhouse gas (warming effect).

Pharmaceutical Aerosols

Pharmaceutical aerosols are one of the most unique and effective drug delivery systems. In pharmacy, the term “aerosol” is generally referred to as the pressurized system that acts by releasing a continuous or metered dose of a fine dispersion of a liquid and/or solid materials containing one or more pharmaceutically active ingredients in a gaseous medium. The contents of a pharmaceutical aerosol dosage may be a fine mist, a course wet or dry spray, a steady stream, or a stable or fast-breaking foam. The key advantage of pharmaceutical aerosols is observed from their ability to offer a localized effect by delivering a small amount of the aerosolized drug particles to the site of action (i.e., directly toward the surface of the airways) resulting in a rapid clinical response. There are mainly two types of pharmaceutical aerosols sprays.

• Space sprays:  These are the aerosols that are used to provide airborne mist with the particle size of the released product less than 50μm.

• Surface sprays:  These are the aerosols that are intended to carry the active ingredients to a surface such as to the skin.

The effectiveness of pharmaceutical aerosol’s therapeutic performance is affected by various factors such as type and characteristics of propellants including vapor pressure of propellants, viscosity and density flashpoint, and also other factors such as type and characteristics of active ingredients, containers, valves, and actuators, along with the percentage of moisture content and mechanism of emitted dose deposition, spray pattern, the efficiency of valve crimping, and measurement of particle size aerosols. Let’s discuss briefly the main types of aerosol systems and aerosols produced by the pharmaceutical industry.

Aerosols systems

• Two-phased system: This system comprises a two-phase mixture of vaporized product concentrate (active ingredient) and a liquified propellant that expels the formulation in spray form. If the active ingredient is directly soluble in the propellent, no additional solvent is used in the formulation; otherwise, co-solvents such as ethanol, acetone, propylene glycol, glycerin, and ethyl acetate are used. These cosolvents also help in reducing the vapor pressure of the system, which results in the formation of larger particles on spraying.

• Three-phased systems: A three-phase system consists of a suspension or emulsion of one or more active ingredients in addition to the vaporized propellants. It is a three-layered heterogeneous mixture consisting of a layer of water-immiscible liquid propellant, a layer of propellant immiscible liquid that contains the product concentrate, and the vapor phase. This type of system is used when the formulation requires the presence of a liquid phase that is not propellant miscible. When the aerosol valve is actuated, the pressure of the vapor phase causes the liquid phase to rise in the dip tube and be expelled from the container. If the product is to maintain the liquefied gas reserved for the latter sprays, the dip tube must not extend beyond the aqueous phase. Sometimes it is desirable to have some liquefied propellant mixed with the aqueous phase to facilitate the dispersion of the spray or to create a foam. In this case, the container should be shaken immediately before use.

• Compressed gas systems: Aerosols using compressed gases as the propellant operate essentially as pressure sprays. The pressure of the compressed gas in the head of the aerosol container forces the product to rise up the dip tube and out of the valve. The use of insoluble gases like nitrogen results in the emission of the product, essentially in the same form as it was placed inside. Also, nitrogen is an odorless and tasteless gas and does not contribute to the taste or the smell of the drug. Other gases, such as carbon dioxide and nitrous oxide, which are slightly soluble in the liquid phase of aerosol products, may be employed when their expulsion with the product concentrate is desired to achieve spraying or foaming.

Inhalation Aerosols

Inhalation therapy was originated as a treatment for the respiratory system’s conditions in some traditional cultures including the Ayurvedic medicine in India, which dates back to around 2000 BCE. It was not until 1956 when scientists invented the metered-dose inhaler (MDI), and inhaled medications reached the status of being safe, effective, and convenient enough for widespread public use. The pulmonary route of drug delivery can provide an excellent alternative to other routes both for local lung disease as well as systemic delivery. The intent of using inhalation aerosols is to produce fine particles and droplets for inhalation through mouth and deposition in the pulmonary tree. The process begins with identifying the target tissue and then utilizing technological parameters such as particle size adjustments through formulation techniques and delivery devices to most efficiently deliver the desired dose. The medical device used for this purpose is known as an inhaler. There is a wide variety of inhalers that are commonly used to treat numerous medical conditions such as asthma and chronic obstructive pulmonary disease (COPD). Medical professionals select the type of inhalers based on the specific needs of the patient, age, coordination, and lung function. Let’s take a look at the most common types of inhalation aerosols.

Examples

• Metered-Dose Inhalers (MDIs): It is the most common type of inhaler, which contains three main components: a metal canister, plastic actuator, and a metering valve. The medication is typically stored in a solution in a pressurized canister that contains a propellant or suspension. On actuation, it delivers a specific amount of medication to the lungs, in the form of a short burst of aerosolized medicine that is usually self-administered by the patient via inhalation. It is most commonly used to deliver medication for asthma patients and patients suffering from other pulmonary disorders. Other medications less commonly used but also administered by MDI are mast cell stabilizers, such as cromoglicate or nedocromil.

• Dry Powder Inhalers (DPI): As the name suggests, it is a device that is used to deliver powdered form medication directly to the lungs (or targeted tissues).  The medication is commonly held in a capsule for manual loading inside the inhaler. The delivery of the medicine is done through a narrow mouthpiece that the patient puts in his/her mouth. Most DPIs rely on the force of patient inhalation to entrain powder from the device and subsequently break-up the powder into particles that are small enough to reach the lungs. For this reason, insufficient patient inhalation flow rates may lead to reduced dose delivery and incomplete deaggregation of the powder, leading to unsatisfactory device performance. Thus, most DPIs have a minimum inspiratory effort that is needed for the proper use and it is for this reason that such DPIs are normally used only in older children and adults.

• Soft Mist Inhalers (SMIs): The soft mist inhaler (SMI) is a newer type of device. It creates a cloud of medicine that one can inhale without the help of a propellant. Because the mist contains more particles than MDIs and DPIs and the spray leaves the inhaler more slowly, more of the drug gets into the lungs. Furthermore,  the mist comes out more slowly and lasts longer in the air than the aerosol produced by MDIs. The medication is released by pressing down a button on the side of the inhaler while breathing in. It requires less coordination while- using and may be helpful for young patients or patients that find the MDI inhalers difficult to use. The bronchodilator drugs tiotropium (Spiriva Respimat) and olodaterol (Striverdi Respimat) both come in a soft mist. SMIs come with a dose counter built-in, so you can see how many doses of medication remain; it also turns red when the inhaler is nearly empty. The device locks itself after all the medication has been used.

• Nebulizers: Nebulizer is a unique inhaler technology that relies on electricity to deliver medication over an extended period of time. They use oxygen, compressed air, or ultrasonic power to break up solutions and suspensions into small aerosol droplets that are inhaled from the mouthpiece of the device. When a human being is under an asthma attack, the airways inside the lungs become narrower, which makes breathing nearly impossible. Under such circumstances, a nebulizer is a more suitable choice than MDIs as a nebulizer can deploy the aerosolized medicine to the deepest parts of the pulmonary system on its own. Moreover, nebulizers are also used for long-term medications of asthma and certain other pulmonary diseases. There are various types of nebulizers, including jet, vibrating mesh, and ultrasonic, which differ in their performance in delivering bronchodilators or corticosteroids for the treatment of respiratory obstructive diseases.

Nasal Aerosols

Nasal delivery is the rational choice of treatment of diseases in the nose and paranasal sinuses such as allergic and non-allergic rhinitis and sinusitis. In some situations, the nasal delivery route is preferred for systemic therapy because it provides an agreeable alternative to injections or pills. Solutions of pharmaceutically active ingredients can be formed into an aerosolized nasal spray that is extremely quick and direct. Many pharmaceutical drugs exist as nasal sprays for systemic administration (e.g. sedative-analgesics, treatments for migraine, osteoporosis, and nausea). Other applications include hormone replacement therapy, treatment of Alzheimer’s disease, and Parkinson’s disease. Nasal sprays are seen as a more efficient way of transporting drugs with potential use in crossing the blood–brain barrier.

Examples:

• Antihistamines

• Corticosteroids

• Decongestants

• Saline

Topical Aerosols

The topical aerosol products are intended for the application of the pharmaceutically active ingredients directly to the skin. They are commonly used in the treatment of burns, minor cuts, bruises, infections, and various dermatological conditions. When an aerosol is designed for topical applications, it is recommended to keep the size of particles between 3-6 μm to avoid the probable adverse effects of accidental inhalation. Most of the topical aerosols are made up of those hydrocarbon-based solutions which are inert, non-toxic, and environment friendly. Developments in topical drug delivery have been incremental, focusing on overcoming problems associated with the barrier properties of the skin, reducing skin irritation rates, and improving the aesthetics associated with conventional topical drug delivery systems. . The non-invasive character of topical drug delivery makes it accessible to a wide range of patient populations and is a highly acceptable option for drug dosing.

Examples:

• Non-steroidal anti-inflammatory sprays

• Antifungal Aerosols

• Econazole Nitrate spray

• Procaine anesthetic

• Non-central analgesic

• Antiinfectives, i.e. antibiotics, antiseptics, chemotherapeutics

Bioaerosols

Bioaerosols (short for biological-aerosols) comprise the group of particles that originates biologically from the terrestrial and marine ecosystems. They consist of both living and non-living components such as metabolites, toxins, or fragments of microorganisms that are present ubiquitously in the environment. Global interests in bioaerosols have increased rapidly to broaden the pool of knowledge on their identification, quantification, distribution, and health impacts (e.g., infectious and respiratory diseases, allergies, and cancer). Biological aerosols are either directly released or carried via turbulent winds to the atmosphere. They play a vital role in the earth ecosystem, particularly in the interactions between the atmosphere, biosphere, climate, and public health. One of the most important functions of bioaerosols includes the dispersal of reproductive units from plants and microbes (pollen, spores, etc.), for which the atmosphere enables transport over geographic barriers and long distances. Let’s take a look at a few types of bioaerosols present in the atmosphere.

Fungi

A fungus is a member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. Fungi come in a variety of forms which include creeping rots, fluffy molds, unicellular yeasts, large bracket fungi, toadstools, and mushrooms. One feature the great majority have in common which is of importance to aerobiologists is that they produce numerous airborne spores and other viable fragments which at times constitute the major particulate component of the air. Fungal aerosols consist of spores and fragments with a diverse array of morphologies. While most fungal aerosols die when they travel through the atmosphere due to the desiccating effects of higher altitudes, some of them can survive their travel despite being exposed to UV radiation. The size of fungal spores varies considerably but, in general, they are bigger than most individual bacterial cells and smaller than pollens. Let’s take a look at few examples of fungal aerosols that are present in ambient confound in the atmosphere.

Examples:

• Cladosporium

• Alternaria

• Penicillium

• Aspergillus

• Epicoccum

• Yeasts

• Smuts

• Rusts

• Basidiomycetes

All these fungi have the potential to cause a medical condition known as allergic alveolitis; however, there are not enough pieces of evidence to support these facts.

Viruses

Viruses are one of the smallest primary biological aerosols. They can be as small as 20 nm in diameter and are not commonly airborne as individuals but more likely to attach to other suspended particles. Many studies have revealed that the presence of viruses as primary bioaerosols is not numerous. However, they are the major concern among health societies due to their contagious properties. Many diseases present in humans, animals, birds, fish, insects, and plants are caused by viruses found in aerosols (one of the possible routes of infection transmission). Many environmental factors may affect virus survival, including temperature, humidity and virus type (lipid and non-lipid enveloped), the presence of surrounding organic material (e.g. saliva and mucus), sunlight (ultraviolet light), or antiviral chemicals. Let’s take a look at few airborne viruses that have caused several epidemics around the globe.

Examples:

• SARS-CoV-2 (Disease: Covid-19)

• Rhinovirus (Disease: Common Cold)

• Influenza (Type A, Type B, H1N1) (Disease: Influenza, Flu, Swine Flu)

• Varicella-zoster (Disease: Chicken Pox)

• Rubeolavirus (Disease: Measles)

• HantaVirus (Disease: Hantavirus Pulmonary Syndrome)

Bacterias

Due to their small size, bacteria have a relatively long atmospheric residence time (on the order of several days or more) compared to larger particles and can be transported over long distances (up to thousands of kilometers). Unlike other bioaerosols, bacteria are able to complete full reproductive cycles within the days or weeks that they survive in the atmosphere, making them a major component of the air biota ecosystem. The survival of bacteria depends on water droplets from fog and clouds that provide bacteria with nutrients and protection from UV light. Airborne bacteria may be suspended as individual cells, but they are more likely to be attached to other particles, such as soil or leaf fragments, or found as agglomerates of many bacterial cells. For this reason, whereas individual bacteria are typically on the order of ~1 μm or less in size, the median aerodynamic diameter of particles containing bacteria lies in the range of ~2 μm to ~4 μm.

Examples: 

• Legionella pneumophila  (Diseases: Pneumonia, LD fever)

• Mycobacterium tuberculosis  (Disease: Tuberculosis)

• Bacillus anthracis (Disease: Anthrax)

• Corynebacteria diphtheria (Disease: Diptheria)

• Yersinia pestis (Disease: Pneumonic plague)

• Mycoplasma pneumonia (Disease: Pneumonia)

Pollen

Among all the primary bioaerosol particles, pollen grains can be among the largest in physical size and represent the reproductive units of plants that contain the male gamete. They can have various shapes with sizes varying from ten to a few hundred micrometers. As a bioaerosol, they can either exist as a complete pollen unit or as fragmented pieces. Pollen can rupture when the humidity is high, and these fragments have been shown to be in the range from 30 nm to 5 μm. The amount of pollen relinquished by individual plants can vary greatly from 1 yr to another. The increased concentration of pollen in the environment is closely linked to increases in asthma cases. They are a major source of wind-dispersed allergens, coming particularly from seasonal releases from grasses and trees.

Uses of Aerosols

Aerosols offer a wide range of products from mass-market goods such as cosmetic and household products, to specific aerosol types dedicated for industrial or medical purposes.

• In the cosmetic industry, aerosols are manufactured as several body care products including hair-sprays, deodorants, shaving foams, hydrating creams, sun-protection moisturizers, etc.

• If we look around, we can find several household products that are based on aerosols technology, e.g., air fresheners, cleaning products, polishing products, anti-static aerosols, insecticides, pesticides, etc.

• Aerosols are also used for calibrating instruments, performing research, and testing sampling equipment and air filters.

• In the industrial sector, there are several applications of aerosols that include both mass-market and professional uses. For instance, several cleaning processes such as engine cleaning, console cleaning, and upholstery cleaning is done with aerosols. Maintenance services such as de-icing and anti-fogging also make use of aerosols in the industries.

• Medical & pharmaceutical aerosols have played a crucial role in the health and well-being of millions of people throughout the world for many years. These products include pressurized metered-dose inhalers, aerosol coolants, aerosol disinfectants, aerosol anesthetics, and aerosol bandages.

• Aerosol technology is also used to deliver paints and several other products in spray form.