To better understand why IAQ is so important for our health, let’s review the ways that toxic chemicals, microscopic particles, and infectious microbes in our environment can enter our body. Substances can gain access to our body cavities through three physical avenues (not including an injury or electromagnetic radiation): direct diffusion through our skin or mucus membranes, inhalation from breathing, and ingestion through swallowing. After either intentional or accidental exposure, chemicals and microbes affect our health differently depending on which tissues are penetrated and on whether or not our immune system has been able to process information from the “invader.” Whether or not our immune system has the opportunity to be “trained” by the substance depends on the molecular characteristics of the element and the integrity of our natural protective barriers.

If our skin, eyes, throat, airways, etc., are exposed to a compound made up of small molecules that are non-polar, and therefore soluble in fat but not in water, the substance can enter deeper tissues by simply progressively diffusing through layers of cell membranes. Pharmaceutical companies depend on this diffusion pathway when delivering medications through skin patches, rectal suppositories, and tablets placed under one’s tongue. If the small, nonpolar compound is harmful — for example benzene or toluene — the only way to control toxicity is to prevent exposure in the first place.

A second category of substances in buildings includes polar (water soluble) molecules, proteins, inorganic mineral dust, microbes, and aerosols. These elements can enter our blood and organs through routes that are directly influenced by IAQ. Inhaled solid particles are usually trapped by “big-eater” immune cells called macrophages. The particulates are then either degraded by macrophage enzymes or remain in our body, sometimes inert and non-problematic, but sometimes causing ongoing inflammation.

When the membranes in our nose, mouth, throat, and bronchial tubes are healthy and well hydrated, inhaled microbes are trapped in a layer of mucus that flows away from our lungs due to the continual upward motion of tiny hairs called cilia. This prevents particulate debris, bacteria, and viruses from settling deep in our lungs, where inflammation or infection could become life-threatening. The entrapped microbes pass into the digestive system, where they can join existing communities of beneficial bacteria known as gut microbiome.

Conversely, when indoor air has low water vapor content (relative humidity less than 40%), the layer of mucus in our respiratory system layer loses moisture during breathing. This causes the mucus to become viscus, and ciliary movement is inhibited. Inhaled particles or microbes are less likely to be trapped and swallowed and therefore have a higher chance of causing lung infections or inflammation. 

In addition to causing bronchial tube or lung disease, substances penetrating deeply have a much higher chance of reaching the blood stream and causing life-threatening systemic disease.

Because of this relationship between indoor air and human physiology, we now know that mechanical engineers are key players in protecting occupants from both acute and chronic diseases.

Here is what you should be aware of:

1. Air is a transmission route for many infectious microbes and particles;
2. Ambient relative humidity of 40–60% supports a healthy immune system; and
3. Just like data servers, valuable paintings, and products, humans need proper humidification.