Did you know? Humidity can amplify allergen levels by increasing dust mite populations and increase airborne mold counts.Humidity Levels and Allergies Florida has been labeled the "Sick Building State". There are more Allergy/Indoor Air Quality complaints in Florida than any other State. If you have ever visited a Florida Hotel room and detected the odor of mold in your room, you probably understand why Florida is frequently mentioned as being a haven for allergy complaints. While it is probably unfair to categorize Florida in this manner, Florida is an ideal laboratory to evaluate allergy complaints and remedies.
Humidity is a primary factor in many allergy complaints and Florida is wonderfully humid. Humidity, itself, doesn't cause allergies, but it certainly contributes to allergen proliferation.  The phenomena of Indoor Air Quality (IAQ) is a fairly new science and still in its early stages of investigation, but a great deal of economic resources are being devoted to its investigation. It has created a plethora of new products, such as ozone generators, air purifiers, electrostatic  whatchamacallits, and other products that may provide dubious claims and limited results. While many of these products may provide limited benefit, the idea that these gadgets can eliminate allergic symptoms is a bit far-fetched.
In examining IAQ Complaints and Allergy complaints, it is difficult to distinguish between the two. IAQ problems mirror allergy complaints. In a typical building IAQ evaluation, between 15-18% of building occupants report "classic" IAQ complaints. If you examine study results of allergic individuals, between 15-18% of the U.S. population suffer from allergies. In each case, whether in our own homes, at school, or at work, there is usually one common factor - humidity. Schools have become a primary emphasis, primarily because of concern for our children, but classrooms are another good laboratory for IAQ research. The average home is approximately 1400 sq. ft. and has 3.2 occupants, The average classroom is about 600 sq, ft. and has about 28 occupants. This heavy occupant loading allows contaminants to load up quickly and the participants in the research make up a widely diverse study population. More importantly, schools are strapped for cash. Cleaning has been dramatically cut back and the ventilation system is operated on a minimum schedule to save education dollars. Each of these factors contributes to poor indoor air quality. The problems associated with cleaning cutbacks are obvious with an increase in contaminant (allergen) loads, but Heating, Ventilation, and Air Conditioning (HVAC) was not quite so obvious until the 1980's. This is an extremely complex issue and volumes have been written about this subject. We will not be able to do sufficient justice to the question of ventilation in this short space and the principal topic for this section is Humidity and Allergy. Here is a brief synopsis. In 1973, ASHRAE reduced the airflow standard to 5 Cubic feet of air per minute (CFM) per occupant to help reduce increased costs related to the Arab Oil Embargo of 1973. (This has since been raised to 15 CFM per student) Reduced airflow allows a number of things to happen. Indoor contaminant levels are not diluted and indoor chemicals are allowed to rise. Chemicals such as carbon dioxide become elevated and children begin to become drowsy and irritable. Learning is negatively affected. Other chemicals may initiate allergy-like symptoms. Add to the mix that schools have reduced HVAC operation to 7-8 hours per day and this elevates levels further. One factor of poor ventilation that probably did not receive enough early scrutiny was the effect of ventilation on humidity levels. Ideally humidity levels should be maintained at 50% rh or less. Until very recently, it was only was "suggested" that levels be maintained at 60% or less. New standards require that levels be maintained below 65% rH. Early studies primarily examined humidity's effect on occupant comfort, but humidity has far more serious effects on occupant health. Outdoor air is a highly diverse mix of airborne particles, chemicals, and biologicals. These contaminants are brought into the building via the ventilation system. As contaminants such as mold are deposited on interior surfaces, humidity levels above 60% allow these molds to become viable and begin active growth. This stimulates an interior source for mold and mold is a primary allergen. Reduced ventilation operation (hours) further exacerbates this issue. Consider the following; 
In the graphs above, simultaneous indoor and outdoor relative humidity readings were taken at an  elementary school in Tampa, FL. Note the hours of HVAC operation (7:00 to 3:00) At 7:00 A.M. exterior humidity levels are at their highest point. The HVAC is turned on and the wet, humid air is pumped into the classroom. As the HVAC system cycles on and off during the day, interior humidity levels are gradually lowered. At 3:00 P.M. interior humidity levels approach ideal levels and the HVAC system is turned off. Part of the function of the HVAC system is to reduce humidity. In the case above, if the HVAC system hours of operation were extended to 12 hours or more per day, humidity levels would remain in the ideal range far longer and biological growth could be arrested. Humidity is the catalyst for biological growth and a primary concern for allergen proliferation. Allergen triggers go far beyond pollen. Biological allergens contribute more to the allergy proposition than previously thought. Primary biological allergens, such as mold and dust mites, require sufficient moisture to remain viable. Consider this. Dust mites live in a virtual desert. They receive all of their nutrient and moisture needs from dead human skin scales that are shed daily. We shed millions of skin scales each day and this dead skin comprises a high percentage of the dust we find in our homes. Have you ever looked at a beam of sunlight within your home and saw particles floating within this beam? A large portion of this dust is our own skin. This skin absorbs moisture from ambient room conditions and this allows dust mites to feed on this softened tissue and obtain both moisture and nutrients. Dust mite allergen is actually the fecal matter that dust mites pass during the course of their lifetime and our pillows and mattresses are loaded with this matter. So essentially, you see, our pillows are loaded with "dead rotting flesh" and "dust mite poop", which is pretty disgusting. I observed a few years ago, that these contaminants in my own mattress or pillow don't bother me, because they are MY dust mites. However, staying in a hotel room in my frequent travels concerns me a bit, because I have no idea of where the contaminants came from. I have since started to pack my own pillow on business trips. Numerous studies have shown that by reducing humidity levels below 50% rH, most dust mites die within a few days. During winter months, the heating season dries the air and the dust mite population is dramatically reduced. See Dust Mites  Beyond dust mites, fungi (mold and mildew) also require moisture. Outdoor air contains thousands of colony forming units (CFU's) of viable mold spores in every cubic meter of air. These mold spores are introduced into the home via the ventilation system. As they settle to a horizontal surface, they are typically dormant. They can remain dormant for long periods until they receive moisture which activates their growth. Fungal spores become active, immediately, when moisture is introduced. Various studies indicate that 14-16 hours of moisture is enough for these spores to become active, sustain life and they can go dormant again when conditions dry up.
As you might expect, humidity is enough of a moisture catalyst for these viable spores to become active. Typically, most experts agree that humidity levels above 65% will allow unrestricted fungal growth, while maintaining humidity levels below 60% will arrest fungal growth. this is one of the primary reasons that the American Society for Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) have  established new standards that require that humidity levels never exceed 65%. Beyond these obvious reasons in which humidity levels affect occupant health, Humidity plays a significant role in psychological health as well. Several studies have been initiated that compared indoor air quality (IAQ) complaint rooms within schools with non complaint rooms within the same school. These studies (above) examined airborne fungi in each room classification (complaint vs. non complaint), surface fungi levels, dust mite allergen levels, humidity levels, cockroach allergen levels, cat allergen levels and a whole host of other allergens within each classroom classification. While the results were confusing at first, (classrooms with very high allergen levels had fewer complaints) the only common denominator between the classifications was that classrooms with higher humidity levels seemed to produce more IAQ complaints than classrooms with heavy allergen content.
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