Humidity is the concentration of water vapor in the air. This is an important and often overlooked variable in maintaining good health. During the winter months, cold temperatures and furnaces lead to dryer air with low humidity.
High and low humidity extremes can trigger nasal congestion or the perception of congestion. Dry air with low humidity can increase feelings of being congested as your sinus membranes dry out and become irritated. The authors of one study found high humidity contributed to nasal patency, or the experience of breathing through clear nostrils.1
Low humidity can also contribute to dry, irritated eyes and it may be a factor in increasing the evaporation of your tears. Colder temperatures and lower humidity also tend to dry out your skin.
Knowledge that humidity plays a role in the rate of respiratory infections is not new. In one study published over three decades ago, researchers found that maintaining mid-levels of humidity could help to lower the rate of respiratory infections and allergies.2
Proper Humidity Improves Human Immune Function
In a paper published in the Journal of Global Health,3 scientists reviewed the literature and proposed that humidity not only may reduce transmission of viral infections, but also play a role in your immune response.
They suggested the increase in viral infections during the winter months is a function of damage to the mucosal barrier by dry air. Within the mucus membranes are glycans, which are chemical structures that are bonded to most proteins. When pathogens enter the body, glycans are involved.
Mucins add another layer of protection. These glycosylated proteins found in the mucosal barriers are a decoy trap for viruses. Once trapped, viruses are then expelled out of the airway. While these barriers are highly effective, they require proper hydration to maintain functionality.
When mucous membranes are exposed to dry air, their protective function is impaired. The results from an animal study4 demonstrated that raising relative humidity to 50% decreased mortality from flu infections. The researchers found that animals that lived in dry air had a reduction in their mucociliary clearance and the ability to repair tissue. They were also more susceptible to disease.
The combination of low temperatures and low humidity is an ideal environment for the spread of viral infections. This plays a prominent role in seasonal changes for viral infections, such as influenza. According to the authors of one study:5
“A key epidemiological study analyzing data collected over 30 y[ears] across the continental United States showed that a drop in absolute humidity, which is dependent on relative humidity and temperature, correlates most closely with the rise in influenza-related deaths.6
Experimental studies in guinea pigs demonstrate that low temperature and low humidity enable aerosol transmission of influenza virus, providing one explanation for the seasonality of viral transmission.7”
The authors of the Journal of Global Health article reported on studies in the U.S. where they found that humidity in residential and commercial spaces are often below 25%. This enhances viral transmission. While humidity during the summer months is often higher, air conditioning limits humidity and air exchange that may result in low indoor air humidity. The researchers suggest:8
“In addition to being a protection against initial infection, functional mucosal barrier is also important in suppression of viral progression in already infected patients. Since many hospitals have very dry air, providing humidified air to patients in early stages of the disease may be beneficial.”
Humidity Affects Ability of Virus to Infect
There are two types of measurements for humidity: One is absolute and the other is relative.9 Absolute humidity is the expression of the amount of water vapor in the air without regard to temperature. The higher the water vapor in the air, the greater the measurement of absolute humidity.
Relative humidity measures water vapor relative to temperature. It’s a measurement of the exact water vapor expressed as a percentage of how much could exist in the air at a given temperature.
In one study10 researchers simulated coughing using mannequins11 and nebulized influenza virus in an examination room. The room was kept at a constant temperature and the samples were collected with the National Institute for Occupational Safety and Health bioaerosol samplers.
Viral plaque assay was used to determine the number of infectious viruses collected. The samples were collected at five different intervals spanning from 15 minutes to five hours after simulated coughing with the nebulizer. They were compared at 20% and 45% relative humidity.
The researchers12 found the viruses retained 70.6% to 77.3% infectivity when the relative humidity in the room was less than or equal to 23%. However, when relative humidity was 43% or higher, the infectivity percentage dropped to 14.6% to 22.2%.
In an analysis of other studies, scientists evaluated influenza virus survival and transmission in absolute humidity. They found absolute humidity also reduced transmission and virus survival, but much more significantly than relative humidity.13
Common Factors in Areas With Low COVID-19 Infections
Influenza has a season based on environmental temperatures, and scientists are hoping the same will be true for COVID-19. In one study,14 data revealed that many of the people who were infected lived in areas with low temperatures and low humidity. Infectious disease specialist Thomas Pietschmann from the Center for Experimental and Clinical Infection Research explained:15
“Viruses have greater stability at low temperatures. This is similar to food that keeps longest in the refrigerator. On cold and usually dry winter days, the small droplets, together with the viruses, float in the air longer than when the air humidity is high.”
Data from recent studies indicate that countries south of the 35 degrees North latitude line have a marked difference in numbers of individuals infected as well as mortality rates.16 Researchers theorize17 this is, in large part, related to people north of the line not getting enough sunlight to retain vitamin D during the winter months.
One group of outliers includes the people of Nordic countries. However, the scientists pointed out while those countries are far north of the demarcation line, vitamin D deficiency is also relatively low, potentially from widespread use of supplements.
Other researchers18 examined climate data from areas reporting significant community infection with COVID-19. They found the numbers in areas along a line between the 30 degrees North latitude line and 50 degrees north latitude were roughly equivalent.
The weather in these areas was also consistently similar. They interpreted this distribution as a function of temperature and humidity that was consistent with how seasonal respiratory viruses react to the environment.
In early April 2020, Mark Alipio published a preprint letter19 describing the results of his data analysis of 212 patient records. Alipio, who undertook the analysis without funding, found that patients who presented with vitamin D levels 30 ng/ml or higher had much better outcomes from COVID-19.
As you would expect, it is far easier to naturally maintain adequate levels of vitamin D south of 35 degrees North latitude. The fluctuations in numbers of people infected and the mortality rates may be a function of both vitamin D levels and humidity.
Indoor Humidity Levels May Protect Against Infection
While outdoor humidity is out of your control, you may have some impact on your indoor humidity levels. Scientists20 found the rate of infection with COVID-19 rose in interior spaces. Interestingly, the highest number of infections were spread in the home (79.9%) followed by transportation (34%), including planes, trains, cars and buses. This demonstrates the need to address the indoor spread of infection.
If you have dry air in your home or workplace, you may experience dry skin, or a dry, scratchy throat. Consider using an inexpensive temperature and humidity gauge so you know the level of humidity at home and work.
There are several ways to increase the humidity in your home. If your workplace has low humidity levels, consider speaking with your employer about reducing the risk of colds, flu or COVID-19 by raising the humidity level to 40% to 60%.
This is the level many experts believe helps moisturize your membranes and reduce the risk of infection.21 If your employer is unwilling to make an adjustment, there are strategies you can use to help maintain the health of your nasal and sinus membranes:
- Consider a vaporizer or room humidifier (see caution below)
- Breathe in steam from a hot cup of tea or coffee
- Boil water on your stove to boost the humidity
- Place bowls of water around your home that help improve humidity as they evaporate
If you decide to use a room humidifier, be particularly careful to keep the humidity levels between 40% and 60%. Consistently high levels of humidity will increase the risk of mold growth. This can have a devastating effect on your health.
The warm, moist environment of a humidifier is an excellent breeding ground for bacteria and fungi, so your machine must be cleaned according to the manufacturer’s instructions at least once every three days.
The water in the reservoir should be changed daily. If you suffer from chronic respiratory infections, allergies and/or asthma, or if you frequently have red and itchy eyes, consider having your home inspected for mold.22
Reduce Your Potential for Infection and Improve Outcomes
There are several ways you can reduce the potential of becoming infected with COVID-19, influenza or cold viruses. If you do become infected, there are strategies you can use to improve your outcomes. In addition to maintaining indoor humidity at 40% to 60%, here are several more suggestions to consider:
Hand-washing — Proper hand-washing is an important strategy to remove harmful pathogens and stop the spread of bacteria. This simple strategy has a significant impact on infection.
Hydration — Keeping your body hydrated is another way of protecting your mucus membranes. It has additional benefits as well as I discussed in “This Could Prevent 3 Million Cases of Degenerative Disease.”
Vitamin D — As I’ve written recently, optimizing your vitamin D levels has proven to help reduce mortality from influenza. The authors of recent studies also found that those with levels of vitamin D above 30 ng/mL have better outcomes from COVID-19.
Diabetes and High Blood Pressure — These two health conditions significantly impact the outcome of an infection with COVID-19. To learn how and what you can do to reduce your potential risk, see “Want to Defeat Coronavirus? Address Diabetes and Hypertension.”
Quercetin and Zinc — Zinc has a known effect on reducing the length of viral infections and quercetin helps to increase the amount of zinc that gets into the cell. I discuss how this happens in “How to Improve Zinc Uptake With Quercetin to Boost Immune Health.”
Vitamin C — This vitamin has a history of reducing the damaging effects of respiratory viruses and is an important part of treating sepsis.
Elderberry — Elderberry protects against viral infections by keeping viruses from entering your cells and replicating. Supplementation with elderberry can shorten the duration of a cold.
Sleep — Sleep plays an integral role in your immune system and has a curious bidirectional link to your gut microbiome. If you need help getting quality sleep each night, here are 33 tips to help optimize your sleep routine.
Gut Microbiome — Optimizing your gut microbiome is a long-term strategy to improve your overall health. The trillions of bacteria living in your gut contribute to increasing or reducing inflammation, depending on whether they are beneficial or harmful bacteria.
A vital first step is to reduce the amount of sugar you eat, whether it’s refined white sugar or metabolized from carbohydrates in your food or beverages. There are other simple strategies you can start implementing now to help protect your health in the years to come. Find more suggestions at “Go With Your Gut.”