We often get asked the question: “What is a ‘normal’ outdoor mold level?”, and my answer is almost always, “It depends.”
What might seem like a simple question is very complex in reality.
Factor One: Geography and Climate.
The USA is a huge country with highly varied climates and natural environments. The most significant factor is the land around you and the types of microbial life that it harbors. The mold outside in Seattle will be much different than the mold in El Paso. It isn’t just large differences in climate and geography that impact outdoor mold levels either.
Micro-differences also have an impact on the outdoor mold levels around your home. What is a normal level of mold surrounding your home could be very different from the mold levels surrounding someone that lives just a mile away! The types and amounts of mold outdoors are also constantly changing over time. When collecting an outdoor sample, it is important to realize you are just catching a snapshot of the levels at that exact moment. You could take a sample on your front porch right now and then again in two hours and obtain different levels. Things as simple as wind direction or if your neighbor mowed their grass recently can impact the current mold levels outside your home.
Factor Two: Seasonality
What is normal on a June day in Denver is not remotely close to what might be normal for a January day in Denver. Generally speaking, we can expect to see a range from ~100-20,000 spores/m3 in the outdoor air. Wind, precipitation, large weather patterns, temperature, and more can all play a role in the outdoor levels of mold!
The importance of collecting an outdoor sample
These countless variables are a large reason why the industry does not have published standards for acceptable mold levels. Since so much of a home’s normal fungal ecology depends on the outdoor air, establishing thresholds of acceptability is a seemingly impossible task. This is why the typical interpretation for mold testing is made by comparing samples from indoor complaint areas to outdoor and indoor reference samples. By comparing the types and concentrations from these samples, an experienced professional can determine whether there is an indoor mold issue or not!
If you have any specific questions please don’t hesitate to reach out!
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