Aerosols are tiny solid or liquid particles suspended in a gas, such as the air we breathe. These microscopic particles originate from various sources, including natural phenomena like dust and pollen, and human activities like vehicle emissions or even breathing, talking, and coughing. Understanding how long aerosols linger is important for comprehending their impact on air quality and potential health implications.
What Determines How Long Aerosols Linger?
The size of an aerosol particle is a primary determinant of how long it stays airborne. Larger particles, often referred to as droplets, are heavier and fall quickly to the ground, typically within seconds or minutes. For example, particles 100 micrometers in size can settle from a height of 8 feet in approximately 8 seconds, while 10 micrometer particles take about 8.2 minutes to settle from 5 feet in still air. In contrast, smaller aerosols, particularly those less than 10 micrometers, can remain suspended for minutes, hours, or even longer, traveling significant distances. Submicrometer particles can persist in the air for several months or more.
Air movement and ventilation also play a significant role in aerosol persistence. Good airflow and adequate ventilation can quickly dilute and carry away airborne particles, preventing them from accumulating. Conversely, in poorly ventilated or stagnant environments, aerosols can linger for extended periods, increasing their concentration. Studies have shown that increased air change rates (ventilation) can drastically reduce the half-life of aerosol droplets, for instance, from 87.2 minutes in stagnant air to 6 minutes with increased ventilation.
Humidity, the amount of water vapor in the air, influences aerosol behavior. In drier air (low humidity), water can evaporate from aerosols, causing them to shrink. Smaller particles, being lighter, can then remain airborne for longer durations and travel greater distances. Some research suggests that certain virus-laden particles, particularly those protected by saliva, can remain infectious for twice as long in drier air compared to more humid conditions. In contrast, higher humidity can cause aerosols to absorb water, grow larger, and settle more quickly. Maintaining relative humidity between 40-60% is often considered optimal for human health indoors and can help minimize airborne spread of pathogens.
Temperature can also influence aerosol persistence indirectly by creating air currents. Temperature differences can lead to thermal instabilities, which in turn can spread particles randomly indoors. Additionally, some viruses, like the flu, may survive longer in colder temperatures, contributing to their spread during colder seasons when people spend more time indoors. For very small aerosol particles, air resistance can counteract gravity for long periods, allowing them to remain suspended.
How Aerosols Affect Indoor Air Quality
Lingering aerosols significantly impact indoor air quality, carrying various substances that can pose health risks. These particles can transport allergens like pollen and dust mites, pollutants from combustion sources, and pathogens such as viruses and bacteria. The persistence of these particles increases the likelihood and duration of exposure for individuals within an enclosed space.
When aerosols carry pathogens, their ability to remain airborne for extended periods increases the risk of airborne disease transmission, particularly for respiratory viruses that can spread through the air.
Beyond health concerns, aerosols can also affect comfort and aesthetics in indoor environments. They contribute to dust accumulation, can create a visible haze, and may carry odor-causing compounds. Therefore, managing aerosol levels is important for maintaining a comfortable and healthy indoor environment. The implications of aerosol persistence extend to various indoor settings, from homes and offices to public transport and healthcare facilities.
Practical Steps to Manage Aerosol Levels
Improving ventilation is a primary strategy for managing aerosol levels indoors. Increasing the rate of air exchange with the outdoors helps dilute and remove airborne particles, reducing their concentration. This can be achieved by opening windows and doors, using exhaust fans, and ensuring heating, ventilation, and air conditioning (HVAC) systems are properly maintained.
Air filtration systems are also effective tools for removing aerosols from indoor air. Air purifiers equipped with High-Efficiency Particulate Air (HEPA) filters can capture a significant percentage of airborne particles, including very small aerosols. Upgrading HVAC system filters to those with higher Minimum Efficiency Reporting Value (MERV) ratings can also improve particle trapping.
Controlling the sources of aerosols is another important step. While some aerosol generation, such as breathing, is unavoidable, reducing unnecessary activities that produce aerosols can help. This includes minimizing aerosol sprays, avoiding indoor smoking, and quickly cleaning up dust-generating activities.
Maintaining optimal indoor humidity levels can also assist in managing aerosols. Using humidifiers or dehumidifiers to keep relative humidity between 40-60% can help. This range can promote the settling of some particles and may reduce the survival of certain pathogens. These combined approaches offer a comprehensive strategy for creating a cleaner indoor environment by mitigating aerosol persistence.