Humidity is a measure of how much moisture is in the air. Our senses are not so refined as to "feel" the moisture content of the air we breathe into our lungs or which touches our skin.
However, the direct and indirect effects of dry air on our health are significant. Prolonged exposure to low humidity can result in health problems for many organs (brain, kidneys, circulation, skin, and eyes) and reduced resistance against chronic and acute airborne pollution, respiratory infections, and allergies.
The link between the viral outbreak and the season has been the topic of much research. The two main factors that contribute to the connection are the changes in environmental parameters and human behavior.
Dry Air Affects Immunity and Viral Spread
When cold, dry air comes indoors and is warmed, the relative humidity indoors drops by about 20%. Such a drop in humidity makes it easier for airborne viral particles to travel.
The hair-like organelles outside of cells that line the body’s airways, called cilia, do not function as well in dry conditions — they cannot expel viral particles as well as they otherwise would.
Mice in an environment with 10% relative humidity had impaired clearance of the influenza virus, compared with mice in an environment with 50% relative humidity. Furthermore, studies have shown that dry air exposure of mice impairs epithelial cell repair in the lung after influenza virus infection. The immune response to viruses is less efficient in drier conditions. One study found that rodents in environments with 10–20% relative humidity “succumbed to influenza virus infection more rapidly than those housed in 50% relative humidity.
However, too much outdoor humidity can also support viral spread. Relative humidity of 40–60% is ideal for containing the virus. Studies in mice also found that an environment of 50% relative humidity contributed to good viral clearance and efficient immune response.
Humidity and Asthma
Many people with asthma find that humid weather makes their symptoms worse. Humidity can affect asthma symptoms. People may notice their asthma symptoms get worse on humid days when there is a lot of moisture in the air. This is especially true when exercising in humid conditions.
High levels of humidity may play a role in asthma symptoms in a variety of ways. The increased moisture can directly irritate the airways, and humidity can increase the levels of other substances in the air that irritate the bronchial tubes, such as pollen and pollution.
High humidity can trigger asthma symptoms, including coughing, wheezing, and chest tightness.
Humidity likely causes asthma symptoms because it triggers bronchoconstriction, which is a narrowing of the airways. Bronchoconstriction may occur because, humid air activates C fibers, which are sensory nerve fibers in the airways. Stimulation of C fibers may narrow the airways and stimulate coughing, which makes it difficult to breathe.
High humidity levels create the perfect breeding ground for mold and dust mites, which often trigger asthma. Higher levels of humidity may also increase air pollution. For example, ozone, which is an air pollutant, rises when humidity levels increase.
Increased levels of humidity also often mean higher temperatures. The highest humidity levels usually occur during the summer months. The combination of heat and humidity can irritate the airways making breathing more difficult. Although everyone with asthma has different triggers, humidity, and asthma symptoms go hand in hand for many people.
Humidity and Skin
Climates with high humidity can be troublesome to our skin. Typically, high humidity occurs along with hot temperatures. When it is hot, our bodies often sweat more than when it is cool. This increased sweating can sometimes cause our skin to breakout.
People who have acne-prone skin are more likely to experience acne flare-up in high humidity. Sometimes, in hot, humid climates, our sweat ducts may close off. This causes moisture to get trapped under the skin which can cause a heat rash. Blisters or bumps characterize a heat rash.
Low humidity generally occurs with colder temperatures. These cold temperatures cause our skin to dry out. Additionally, the cold winds with more freezing temperatures can also take moisture and oils out of our skin. This can cause our skin to become dry or even cracked. Neither of these is pleasant conditions.
Dry skin can be uncomfortable and itchy. Cracked skin can be very painful. It can also be embarrassing if the cracks are on the part of your body that others see, such as your hands or arms.
Some people with pre-existing skin conditions, such as eczema, may experience flare-ups during the colder months due to the low humidity and lack of moisture. Again, this can lead to the skin being dry, itchy, or painful.
During the cold winter months, most people have their heater running. This also contributes to lower humidity and further dries out our skin.
Humidity and COVID-19
Low humidity increases COVID-19 risk and dry air could prompt further disease spread. Lower humidity can be defined as dryer air. The estimate is about a 2-fold increase in COVID-19 notifications for a 10 percent drop in relative humidity.
Humidity affects viral spread in three ways: droplet size, how viral-loaded aerosols float for hours, and stay viral on landing surfaces. When the humidity is lower, the air is drier and it makes the aerosols smaller and aerosols are smaller than droplets. When you sneeze and cough those smaller infectious aerosols can stay suspended in the air for longer. That increases the exposure for other people. When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker.
This suggests the need for people to wear a mask, both to prevent infectious aerosols escaping into the air in the case of an infectious inpidual and exposure to infectious aerosols in the case of an uninfected inpidual.
Dry air appears to favor the spread of COVID-19, meaning time and place become important. Accumulating evidence shows that climate is a factor in COVID-19 spread, raising the prospect of seasonal disease outbreaks.
The outcome of the COVID-19 pandemic may significantly depend on levels of humidity, indoors and outdoors. The common cold and flu reach epidemic proportions during winter. The key outbreaks of SARS-CoV-1 and SARS-CoV-2 — the viruses that cause SARS and COVID-19, respectively — have also occurred in the winter.
Specifically, differences in temperature and humidity affect how stable and transmissible viruses are. Cold, dry, unventilated air may contribute to the transmission of influenza in the winter.
Ninety percent of our lives in the developed world are spent indoors in close proximity to each other. Dry rooms and air-conditioned indoor spaces hike Covid-viral infection. Relative humidity strongly influences the spread of viruses among people indoors, especially in dry rooms.
The role of humidity seems to be extremely important to the airborne spread of COVID-19 in an indoor environment. Public buildings should have at least 40% humidity indoors, capped at 60%, to reduce viral-spread risks for occupants.
Maintaining relative room humidity at between 40% and 60%, like the opening of windows can also reduce the absorption of viruses through a person's nasal passages. Dry air also makes the mucous membranes in our noses dry and more permeable to viruses.
The Northern Hemisphere's approaching winter meant higher risks for millions of people in heated rooms. Colder outdoor air is typically sucked indoors via air-conditioning systems. Heating that inflowing air to a comfortable temperature would significantly lower the indoor relative humidity, which creates an extremely dangerous situation for indoor residents, particularly during the COVID-19 pandemic.
Singapore and Malaysian studies, warned tropical residents to avoid extreme cooling systems because resulting dried indoor airflows would also promote more COVID viability.
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