Ventilation & Miasma Theory

Dec 22, 2020

Because of Covid-19, there has been a newfound interest in ventilation amongst the general public because we know proper ventilation can reduce disease transmission. In previous blog posts, we have also discussed the specifics on what the ventilation targets should be for a building… but how did this come to be? We have novel information about ventilation now but what did society believe was the cause of sickness in the past? One prevailing theory was called miasma theory.  This blog post will discuss this theory and how ventilation played a role. 

Germ Theory Vs. Miasma Theory 

Currently, we know that specific germs, also known as pathogens, can cause sickness. For example, SARS-CoV-2 virus is the culprit for the global pandemic we are weathering through at the moment.  A person infected with coronavirus can transmit this virus to others. This process in a nutshell is what germ theory is all about. A specific germ in an infected person is contagious and can cause a specific sickness. Germ theory came to gain support in the late 19th century (Cite).  Prior to that, miasma theory ruled. 

Miasma theory held that “bad air” was causing sickness.  Bad air in this context referred to odors coming from “putrid substances” like the “offensiveness of the sweats and other excretions, the livid spots, blotches, and mortifications” from a very sick person (Cite). Miasma theory was popular in the Middle Ages during the Black Plague. A key takeaway from miasma theory is that while individual pathogens were not believed to cause specific disease, people did associate air quality surrounding sick people and rotting garbage with disease transmission. 

Improve Ventilation

Miasma theory might have incorrectly attributed “bad air” as the source for sickness but the basic hypothesis for poor air quality causing negative health effects was there. This information enabled engineers and city planners, among other professionals, to consider how the environment can impact public health, specifically how better ventilation can prevent disease spread. 

The popular use of radiant heat in buildings at the beginning of the 20th century might be an example of engineers designing to address bad air. If you have ever lived in a building or home with radiant heat you may know from first hand experience that they can really warm up a room, even making a living space TOO warm. Turning down the heat emanating from a radiator is typically not an easy flick of a thermostat. The simpler option is to crack open a window.  Some researchers have suggested that this was an intentional engineering design to improve ventilation during the winter months (Cite).  It’s debatable if radiant systems had miasma in mind, but it serves as a good recommendation either way if you live with radiant heat! As the old saying goes, dilution is the solution to pollution.  I know I would crack windows open in the dead of winter when I lived with radiant heat. 


Science is constantly evolving.  It comes from experiments, new discoveries, new data, and most importantly past scientific research.  A good example is the miasma theory.  While this theory was incorrectly hypothesizing that “bad air” from “putrid sources” was causing health effects, it did identify a relationship between sickness transmission through the air.  From this information, germ theory developed which we now follow and more clearly understand airborne pathogen transmission. Science is not intended to be purposely confusing, but it is constantly changing which can make it difficult to keep up with the latest information.  This means that we as society must prioritize scientific literacy and be flexible when recommendations change due to new information.