Winter Months Challenge

Winter is almost here and new challenges to clean indoor air are upon us now. In spring and summer we worry about outdoor allergens like pollen and also about pollution created indoors from cooking, and smoking, and VOC’s. Air conditioning keeps us comfortably cool in the noon and afternoon hours. Opening windows in the morning or late evening to let fresh air in insures that we have fresh outdoor air and meets the recommended ventilation rates. Air purifiers add another level of insurance.

But in the winter months’ ventilation is more difficult. We tend to seal the home tighter, add insulation to windows and doors to prevent heat losses and reduce heating costs. But these actions create new problems.

Lack of adequate ventilation allows pollutants to accumulate inside the house and operating furnaces add to the level of CO2 and even increase the risk of carbon monoxide poisoning.

We have to be more vigilant. Also, the holiday season with the added fragrances, to make the home smell “nice”, add to the problem with the increased emission of VOC’s.Click here to see my recommendation on a very efficient HVAC filter.

Importance of Ventilation

Ventilation is still necessary, we must expel the stale air and allow the intake of fresh air. Ideally we should have installed a heat recovery ventilator (HRV) that extracts heat from the stale air that is exhausted from the house and transfers it to the incoming fresh, but cold, air. It is a heat exchanger where the exhaust air warms up the incoming supply air. These systems may be expensive to install but will provide energy savings since the furnace will cycle on less frequently. However, even if you do not have an HRV in your home you still must have ventilation.

But I want to stress that you must have CO, carbon monoxide, detectors in your home since malfunctioning furnaces can be lethal.

Relative Humidity – Comfort or Health

An important factor for comfort in our homes is the level of relative humidity. Studies show that the ideal relative humidity is between 30% and 50%. This is the “goldilocks” level.

Low humidity levels prevent dust mite infestations, mold and mildew growth. It also reduces out-gassing of VOC’s.

High humidity levels can create allergic reactions to mold and dust mites. Growth and musty odors in the home are also apparent. However, it is not always easy to achieve the optimum levels. It is especially difficult during the winter months’.

The challenge of low humidity

When the outside temperature is low the air can carry less water vapor. The result is that the incoming fresh air that you need for adequate ventilation is drier air. Also, once indoors the humidity of the air is further lowered as the furnace removes some vapor from the air. If the humidity is too low the mucous membranes in the nose and throat dry out. In addition to discomfort we become more susceptible to colds, influenza and respiratory illnesses.

The challenge of high humidity

Too much humidity promotes mold growth and dust mite propagation which pose a health threat especially to allergy and asthma sufferers. In addition to the health problems too much humidity can cause condensation on windows, moldy bathrooms and structural damage to the home in hidden areas in the walls or basement where rot may develop.

So what is the ideal indoor Humidity Level?

It really depends on where you live. The outside temperature determines the optimum relative humidity indoors. For example the recommended indoors humidity in most Canadian cities is 35% in the winter and 50% in the summer. {1}

In Minnesota the Department of Public Service recommends the following:

– If the outside temperature is 20 to 40 degreesF humidity indoors should not be more than 40%.

– If the outside temperature is 10 to 20 degreesF humidity indoors should not be more than 35%. {2}

What Should We Do?

Well, the first thing is to find out what is the relative humidity inside our home. To do this we should buy a hygrometer. This is a low cost instrument you can buy at most Home Improvement stores or on line. This instrument measures the relative humidity.

If you find that humidity is too high you should increase the amount of outdoor air supply to achieve the desired balance.

If your humidity is too low you may want to buy a steam humidifier that ideally would be connected to the ventilation system. Steam is the preferred choice for humidifiers not only for its effectiveness at low air temperatures but also for its airborne bacteria fighting benefits. The Canadian CSA HVAC Standards for Healthcare Facilities only allow steam humidification systems.

Insulation of windows is very important. If possible, we should install double-glazed windows to avoid excessively cold inside glass temperature. This is where condensation will occur.


In the winter months’ we should continue all the measures to improve indoor air quality that we implemented in the previous seasons. But we should be especially vigilant of the pollution that we create indoors since we will be staying indoors longer hours and we will make our home air tight to avoid heat losses.

We should limit the use of fragrances and most important we should monitor levels of carbon monoxide. Indoor Air Quality in our homes is our individual responsibility. My recommendation is that you install an Air Purifier in the kitchen and in the bedroom.The above Air Purifier that I recommend is also an air quality monitor.


{1} A report of the Federal-Provincial Advisory Committee on Environmental and Occupational Health, Technical Guide to the Investigation of Indoor Air Quality in Office Buildings, Department of National Health and Welfare, Cat. No. H46-2/93-166 (Canada)



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