Mold spores are naturally occurring and are present both indoors and outdoors. When outdoor mold levels are high, mold spores can easily find their way into buildings through open windows, doors, and ventilation systems. Intuitively, we believe that there should be a strong correlation between the concentrations of outdoor and indoor mold. Whether or not this is true will be discussed below.
On the other hand, indoor mold growth is not solely determined by outdoor mold concentrations. Other factors, such as indoor moisture levels, ventilation, and cleanliness, also play a role in determining indoor mold concentrations. In the second part of the discussion, we will try to define what constitutes a “high” mold concentration and how it is affected by the outdoor mold level.
Over the years, LCS Laboratory has accumulated statistical data about the effect of outdoor mold concentration on indoor mold. In multiple experiments, pairs of indoor and outdoor samples were collected between 2014 and 2023 in different parts of Canada, from residential and office buildings. The independent consultants were advised to keep the doors and windows closed during the sampling and follow ASTM 7391 method (Air-o-Cell technique).
In this article, we present our observations on the effect of environmental outdoor mold on the concentration of mold in indoor air. Since there are no official criteria for what concentration of mold should be considered “normal” and what level should be considered “high”, we will offer our own criteria for mold evaluation.
The compilation of historical data is presented in the chart below.
The black line on the chart represents the median mold concentration among all tested houses. It is interesting to note that there is no correlation between outdoor and indoor mold, as indicated by the coefficient of correlation R2=0.005. Apparently, variations in indoor mold concentration are mostly affected by internal factors like temperature, humidity, and water damage, and less so by the effect of mold that was brought in from outside. The slope of the trend indicates that only 5%-15% of the outdoor mold finds its way into the indoor air, which is quite insignificant.
In order to define what constitutes “low” and “high” mold concentrations, we divided all historical observation data into four equal segments:
- Low indoor mold concentration is found in 25% of the houses with the lowest mold content. These results are located below the green line on the chart.
- Moderate mold level is found in 25% of the houses that are still below the Canadian median value. The results are found between the green and black lines on the chart.
- Elevated mold level describes the next 25% of Canadian houses that are above the median value. This segment can be found between the black and brown lines on the chart.
- High mold concentration describes the last 25% of the houses with the highest mold concentration (above the brown line).
This classification does not address the health effects of mold, nor does it talk about mold outbreaks. Instead, it separates the houses by the level of mold contamination. The tabulated results of this classification will be published later.
If you would like to evaluate mold concentration in your home or office, please contact LCS Laboratory to request our services.
