Fog is a natural phenomenon: a cloud of water vapor condensed close to the Earth’s surface. While pure water fog is harmless, it poses a danger because it absorbs and concentrates airborne contaminants. This process essentially turns the cloud into a chemical or biological soup. The severity of this problem was demonstrated by the Great Smog of London in 1952, which caused an estimated 10,000 to 12,000 premature deaths. The term “smog,” a contraction of smoke and fog, highlights how this natural condition becomes a serious health hazard when interacting with human-generated pollutants.
How Fog Concentrates Air Pollution
Fog becomes a health risk because its microscopic water droplets efficiently scavenge and capture pollutants from the surrounding air. These droplets require a surface to form on; in polluted environments, fine particulate matter (PM) and other aerosols serve as condensation nuclei. The fog absorbs tiny airborne particles, such as PM2.5 and PM10, concentrating them into a liquid form that is more readily inhaled deep into the lungs. This pollutant-laden fog is often exacerbated by a temperature inversion. During an inversion, a layer of warmer air traps cooler, denser air and fog close to the ground, preventing pollutants from rising and dispersing.
This trapping mechanism prevents the dilution of contaminants, leading to a rapid build-up of gaseous pollutants like sulfur dioxide and nitrogen oxides. Once captured within the fog droplets, these gases can undergo chemical reactions, forming stronger acids. For instance, sulfur dioxide can react with water to form sulfuric acid, creating a highly corrosive mix that is far more concentrated than typical acid rain.
Acute Effects on Respiratory and Cardiovascular Health
Inhaling the concentrated particulate matter and acidic aerosols present in polluted fog can immediately strain the respiratory and cardiovascular systems. The fine particulate matter, particularly PM2.5, bypasses the body’s natural defenses and penetrates deep into the pulmonary alveoli. This triggers an inflammatory response in the lungs, exacerbating pre-existing conditions like asthma and chronic obstructive pulmonary disease (COPD). Acute exposure leads to symptoms such as coughing, wheezing, and irritation of the mucous membranes in the eyes and throat.
High concentrations of pollutants can cause acute respiratory illnesses, including severe bronchitis and pneumonia. The cardiovascular system is particularly vulnerable to this stress, as the body’s reaction to inhaled toxins creates systemic effects. Fine particles entering the lungs stimulate the autonomic nervous system, which can result in vasoconstriction, increased heart rate, and elevated blood pressure. The systemic inflammation and oxidative stress triggered by the pollutants increase the risk of acute cardiac events like heart attacks, strokes, and arrhythmias.
Transport of Biological Agents and Allergens
Fog droplets serve as an effective transport mechanism for biological contaminants, in addition to industrial byproducts. The moisture within fog provides an ideal environment for the sustenance and dispersal of airborne microorganisms. Fog scavenges micro-organisms, including bacteria, fungi, and mold spores, increasing their diversity and carrying them over long distances. The droplets’ moisture allows these biological agents to remain viable longer than they would as dry aerosols, increasing the likelihood of exposure and potential for respiratory or skin infections.
Fog also efficiently captures other biological aerosols, such as plant allergens. While large pollen grains may not be easily concentrated, smaller pollen fragments and organic debris are readily absorbed by the fog droplets. The resulting mixture of chemical irritants and biological allergens compounds the risk for individuals with allergies or respiratory sensitivities.
Steps to Minimize Health Exposure
Indoor Protection and Monitoring
The primary defense during periods of polluted fog is to monitor the local Air Quality Index (AQI). When the AQI is forecast to be in the “Unhealthy for Sensitive Groups” range (101 to 150) or higher, limit time spent outdoors. Sensitive populations, including children, the elderly, and those with heart or lung conditions, should avoid all strenuous outdoor activity. Keeping indoor air clean is also necessary, as outdoor pollutants can seep into buildings. This involves keeping windows and doors closed and using a portable air purifier equipped with a high-efficiency particulate air (HEPA) filter, which is effective at removing concentrated PM2.5.
Outdoor Precautions
If outdoor exposure is unavoidable, wearing an N95 or N99 respirator mask provides protection against solid particulate matter. These masks filter out at least 95% of airborne particles but must be fitted snugly to the face to be effective. Note that these masks offer little protection against gaseous pollutants. Limiting activities that increase deep, rapid breathing, such as strenuous exercise, is also advisable to reduce the total dose of inhaled contaminants.