Humidity: Plants, Health and Our Homes

There are a few suggestions that come up year after year for taking good care of our houseplants during the winter season: maximize natural light, add artificial lighting, stop fertilizing, monitor temperature and increase humidity. This last recommendation is important not only for our plants, but also for our health and the condition of our homes. And as in many cases, too much is like not enough. It’s all about finding the right level of humidity!

If you do your research, you’ll probably be told to keep relative humidity between 30 and 50%, but where do these figures come from, and are they accurate?

Humidity and Plants

We often talk about photosynthesis in horticulture, but what exactly is it? Simply put, the leaves of plants (and a few other organisms, including algae) absorb carbon dioxide and water from the air and, with the help of solar energy, convert them into glucose and oxygen. This glucose feeds plants and is used to form new cells. It thus enables plants to grow.

Water is essential to this process, and when the air is very dry, plants lose more than normal. To protect themselves, they close their stomata, small openings in the leaves that allow gas exchange. No gas exchange, no photosynthesis. (Cacti work a little differently, of course. Their fleshy stems store water and a waxy coating on their surface helps retain it).

Typical symptoms of a lack of atmospheric humidity are wilting, slower growth, smaller new leaves and leaves that are curled up or have browned tips. Plants in a state of distress will also be more prone to disease, which explains their prevalence in winter.

Too Much Isn’t Any Better!

When the air is very humid, plants don’t absorb the water in their substrate, nor the minerals it contains. In this case, signs of deficiency may appear. However, this is unlikely to happen to you, especially in winter!

The majority of houseplants are of tropical origin and prefer a relative humidity of 60-80%, sometimes even higher. These same tropical plants can tolerate humidity levels of between 40 and 60%, which is generally a good level for plants of subtropical or temperate origin. Below 40%, you’ll start to see symptoms of dryness in some plants, and below 20% few will survive for long, apart from cacti and succulents. In short, keeping the relative humidity level between 40% and 60% is adequate for most houseplants.

Relative Humidity

We often hear that heating dries out the air, but the opposite is true. It’s the cold that reduces the amount of water in the atmosphere. You see, the colder the air, the less water it can hold, and vice versa. For example, at 20°C (68?), air can hold 17.3 grams per cubic metre of air; at 10°C (50 ?), it’s more like 9.39 g/m3; and at freezing point, it’s 4.89 g/m3. When it’s very cold, say -20? (-4?), the air can’t contain more than 1.05 g/m3!

When cold outside air enters our homes, it contains very little water. As it warms up, its relative humidity is very low.

Relative humidity is simply the percentage of the air’s maximum water capacity, hence its relativity. 1 gram of water at -20? (-4?) is close to 100% relative humidity, while at 20? (68?) it’s around 6%.

This maximum capacity is known as the dew point, because when it exceeds 100%, condensation occurs. This explains dew. Warm daytime air drops in temperature at night and can contain less water. When it exceeds its maximum capacity, the water condenses and dew forms.

Humidity and Human Health

The Regulation respecting occupational health and safety and the Canadian Centre for Occupational Health and Safety (CCOHS) have set the minimum humidity level in a building at 20%, to prevent skin and mucous membranes from drying out. In humans, symptoms of excessively dry air include dry eyes, nose, throat and skin.

Above 60-70% humidity, conditions are conducive to the growth of mold, bacteria and dust mites, which can have an impact on our health. We might conclude that with humidity levels between 40 and 60%, plants and humans will be comfortable all winter long.

Humidity and Our Homes

And if only it were that simple! I can well imagine a building engineer among the readers raising his hand: “60% humidity in winter is really too high for our houses!”

In cold weather, when warm air, which contains a lot of water, meets a cold surface, such as a window, the air temperature drops and so does its capacity to hold water. That’s why we often see condensation on windows in winter.

Imagine that the air in a home is at 20? (68?) with a humidity level of 60%. It therefore contains 10.38 g/m3. Let’s say it’s -20? (-4?) outside and the window temperature drops to 10? (50?). Remember that at this temperature the air can hold no more than 9.39 g/m3 and therefore exceeds the dew point and condensation forms on the windows. In the long term, these accumulations of water could harbor mold and other undesirables.

What Problems Can Arise With Incorrect Humidity Levels?

The situation is worse in poorly insulated buildings, where the cold penetrates the walls. When warm, moist indoor air comes into contact with this cold, condensation forms in the building structure, out of sight. This can lead to health problems for the people who live there, as well as structural problems for the building itself. In addition to mold, corrosion and other deterioration can be linked to humidity.

On the other hand, too low a relative humidity can lead to static electricity problems, cracks in paint, and shrinkage of furniture and wooden floors.

For these reasons, it is recommended to lower the humidity level in our homes in winter. Health Canada suggests keeping the level around 30%. The Canada Mortgage and Housing Corporation (CMHC) is more specific: don’t exceed 45% relative humidity in winter, and when the mercury drops below -10? (-4?) outside, lower it to 30%, and even 25% in extreme cold.

To Your Humidifiers!

All in all, the suggestion to keep humidity between 30 and 50% is generally a good one, but this level of relative humidity is not suitable for all situations. For the well-being of everyone – people, plants and buildings – a little humidity will do, but if you want more, take a trip to a tropical destination or visit a greenhouse!

With the holiday season just around the corner, wouldn’t a humidifier and hygrometer make great gifts for the houseplant lover?