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How to Measure Humidity & Prevent Mold Growing in Your Home

How many times have you heard a family member, friend or co-worker say, “It’s a dry heat.” In Phoenix, the outside air temperature is closely related to the humidity, but inside our Phoenix homes and businesses, tiny micro-environments can cause humidity to wreak havoc. Below we discuss humidity, indoor humidity, how mold grows inside, and how to improve indoor air quality.

What is Humidity?

You may have a hunch about humidity, especially as we deal with it in the Valley and throughout the Southwest. While Phoenix air quality is not great, at least the humidity is low… usually. Knowing exactly what humidity is – or more scientifically, what relative humidity is – can help you understand how to deal with it.



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Relative humidity, according to the National Weather Service, is “a ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.”

Saturated Air & Absolute Humidity

Saturated air is simply air that holds all the water vapor (evaporated water) it can. At a given temperature, meteorologists and other scientists know exactly how much water the air can hold (hotter air holds more water). Saturated air is expressed as absolute humidity. For example, air at freezing (32 degrees) holds no water vapor, so its absolute humidity is zero, while air at 90 degrees can hold 0.031 pound of water for each pound of air.

The all-time hottest day ever in Phoenix, according to Knoji, was June 26, 1990, when the official air temperature reached 122 degrees, at which point the air could have held 0.086 pounds of water in a pound of atmospheric air, but that day the peak relative humidity was only 20 percent, according to the gurus at Weather Underground.

That means, as everyone says, “It’s a dry heat.” That day the air only held 0.001 pounds of water in every pound of air.

Why Relative Humidity Matters

Getting past absolute humidity and saturated air, we arrive at the kind of air we humans (and central air conditioners) deal with every day. Why are you not miserable when Phoenix daytime temperatures routinely hover in the high 90s? And why are you feeling drained and soaked in New Orleans when the temperature is only in the high 80s? The relative humidity is the key difference.

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The more water vapor in the air, the higher the relative humidity (the closer to saturated air). New Orleans becomes a steam bath and Phoenix is (as they say) a dry heat. In New Orleans, your skin cannot shed heat through evaporation of perspiration, so you feel miserable, hot and cranky.

In Phoenix, the air is so dry your perspiration evaporates as soon as your pores exude it and you do not feel miserable, hot or cranky. You can get dehydrated quickly, of course, so keep drinking plenty of water.

This is not to say Phoenix has never had insufferable humidity. Arizona State University has charted high temperatures and relative humidity back to 1898, and the city has had one occurrence of 100 degrees and relative humidity of 45 percent. Now that was a miserable day!

Indoor Humidity

Now that you know how to measure humidity, where should you be concerned about it inside your Phoenix home or business? The air in the Valley is naturally not humid except during the summer monsoon season, so water vapor sources indoors will generally stem from human activity.

Look in low, water-related places around your home:

  • Basement;
  • Bathrooms;
  • Seldom-used rooms;
  • Inside ductwork;
  • Near your home’s hot water heater, central air conditioner, clothes washer or dishwasher;
  • Any spot below ground level.

A sure sign of high, localized humidity is the appearance of mold and mildew.

Mold & Mildew

Any respectable Phoenix homeowner or business owner is interested in preventing mold growth because some types of mold can be harmful to humans and pets. Mold prevention goes hand in hand with mildew prevention, but the two annoyances are not the same.




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Mold spores are everywhere in our air. We breathe them in harmlessly; they settle on surfaces constantly. Only when moisture on a surface provides an oasis for a mold spore can mold really start growing rampant in your home. The spores in molds can reproduce sexually or asexually, and once they have a foothold, they grow fast. They grow through filaments, called hyphae, and can spread across vertical and horizontal surfaces in as little as two days. Molds are types of fungi and are characteristically black, blue, red and green in color.

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Mildew, on the other hand, is always white. It is essentially another type of fungus and, like mold, produces an unpleasant odor. Generally you will smell mildew before you spot mold, but mildew is a precursor to mold.

Black Mold Prevention

Among molds and mildew, black mold is especially hazardous to humans and pets. Stachybotrys chartarum, or black mold, is often singled out as especially dangerous, but the Centers for Disease Control (CDC) recommend treating all molds with “the same with respect to potential health risks and removal,” because any mold can cause a reaction.

Health issues related to inhaling mold, says the CDC, include:

  • Upper respiratory tract symptoms;
  • Cough;
  • Wheeze;
  • Asthma symptoms in people without asthma;
  • Hypersensitivity pneumonitis in individuals susceptible to that immune-mediated condition.

In your hunt for mold around your home, you will likely not become ill from a single exposure, so finding it should not make you worry. Removing the mold means removing the source of water that allowed the mold to grow.

The key is keeping your entire home’s or business’s humidity low (below 50 percent) around the clock. Central air conditioner does a superb job of this, as it removes water vapor from the treated air and sends it out of your house to evaporate outside.

Make Sure You Have Adequate Ventilation

Another preventative is adequate ventilation in bathrooms and the kitchen. Exhaust fans can supplement the air circulation provided by your central air conditioner.

The CDC also recommends common-sense steps:

  • Add mold inhibitors to interior paints;
  • Use mold-killing cleaning products in bathrooms and kitchen;
  • Do not carpet bathrooms;
  • Remove and replace flooded rugs and carpets.

Indoor Air Quality (IAQ)

You can follow two typical paths to having your indoor air quality tested for mold and other pollutants:

Hiring a professional HVAC repair company to assess your home’s or business’s indoor air quality (IAQ) is the only way to gain assurance that mold is not a problem and that your family is safe. By hiring trained professionals with air quality testing certification, you can get trustworthy results.


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Technicians have undergone specialized training, use more sophisticated testing equipment and possess the ability to test across a wider range of indoor pollutants. The report provided by trained experts will be far more reliable and actionable than a printout sent weeks after you mail a send-away kit.

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If you suspect mold or mildew is forming in out-of-the-way places inside a building you, your family or employees use, contact a local, reliable HVAC contractor to perform an IAQ test. Once the air is tested and results are shared with you, you can decide how best to proceed.

Humidifier Health Benefits & Solutions

Getting the IAQ test done professionally could be the end of your issue, if the report comes back negative (no mold, no pollutants). With Phoenix’s air and typical indoor appliances, though, expect to get some readings that could cause concern.

A professional HVAC contractor like ACE Home Services can not only test your IAQ, we can also provide solutions to problems such as presenting to you humidifier health benefits.

Perhaps the easiest solution to improving your IAQ is proper service and filtration on your central air conditioner. As systems age, they build up dust and inefficiencies throughout, so a thorough cleaning could get to all major elements:

  • Evaporator coil;
  • Condensate drain line;
  • Ductwork;
  • Permanent filter (if so equipped);
  • Air handler.

A clean, energy-efficient central air conditioner can remove humidity and lower levels throughout your home, so no bathroom, basement corner or kitchen area gets humid enough to allow mold and mildew to form.

What Else Can be Done?

Other options include air purification systems. These and highly efficient filters can remove chemical fumes, pet dander, mold and mildew from your air.

Have ACE Home Services Test Your Air Quality Today

At ACE Home Services we want to do more than simply alert you to the potentially bad air in your home or business. We want all our Valley residents to enjoy happier lives with healthier, properly humidified air.

Please contact us today to learn how we can measure and improve your indoor air quality in your home or place of business. When you reach out to us, be sure to ask about our special offers on just about any home issue, including air conditioning, air purifying and duct cleaning.

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