The Necessity of Two Ambient Samples

Do you know how mold testing companies like Above Environmental determine whether the mold content in a home or business is elevated—and why some methods are more reliable than others?

There are no federal, state, or local statutes that define what constitutes an acceptable level of mold in indoor air.

Instead, we establish our baseline by sampling the exterior, or ambient, air. This gives us a reference point to compare indoor samples against. Ideally, indoor air should contain fewer total mold spores (measured in spores per cubic meter of air) and a similar mix of spore types to what’s found outdoors.

In future posts, we’ll discuss some of the challenges involved in making these comparisons. For now, though, let’s focus on why it’s important to take two exterior samples instead of just one.

Most industry groups recommend that hygienists collect two outdoor samples—typically taken at the road, sidewalk, or other open area—yet many companies only take one to reduce lab costs. At Above Environmental, we always collect two exterior samples unless a client specifically requests otherwise.

This matters because outdoor mold concentrations can fluctuate significantly over short periods of time. We take our first outdoor sample when we arrive and a second when we leave the property. We’ve seen mold levels vary by 20%, 40%, even 60% between these two samples.

That difference can determine whether a home or business is considered “elevated” and in need of remediation—or not. So when you hire an environmental hygienist to test your property for airborne mold, ask whether they plan to take one or two exterior samples. It can make all the difference.