What Snow and Cold Temperatures Mean for Mold Testing Results

Winter brings a unique challenge for mold testing—especially in regions where snow, freezing temperatures, and prolonged cold are the norm. While mold problems don’t disappear in winter, how we test for mold—and how we interpret results—must change.

Inclement weather can influence spore behavior, sampling equipment performance, and the reliability of outdoor baselines. Below is what homeowners and inspectors should understand when mold testing is performed during snowy or freezing conditions.

How Cold Weather Affects Mold Spore Concentrations

Mold spores are always present in the environment, but temperature and moisture directly influence their activity and detectability.

Outdoor Spore Suppression

• Cold temperatures and snow cover suppress outdoor spore counts

• Snow physically traps spores and organic debris

• Many fungi enter a dormant state during freezing conditions

As a result, outdoor air samples often show unusually low spore concentrations in winter, sometimes approaching zero.

Indoor Conditions Still Matter

While outdoor spores may drop, indoor environments remain active:

• Heated homes create stable temperatures

• Moisture sources (leaks, condensation, humidifiers) persist

• Mold growth inside walls, attics, crawlspaces, or HVAC systems can continue year-round

This means indoor mold concerns do not disappear in winter, even if outdoor air appears “clean.”

Cold Weather and Air Sampling Equipment: A Hidden Risk

One of the most overlooked winter issues is equipment exposure—specifically spore trap cassettes.

Why Temperature Matters for Cassettes

Most air sampling cassettes contain an internal gel or adhesive pad designed to:

• Capture airborne particles

• Preserve spore morphology

• Allow accurate microscopic analysis

Extreme cold can compromise this gel by:

• Causing hardening or cracking

• Reducing adhesive efficiency

• Altering particle capture dynamics

If cassettes are:

• Stored in freezing vehicles

• Left outside before sampling

• Exposed during transport

…the integrity of the sample may be compromised, potentially skewing results or undercounting spores.

Best Practices for Inspectors

• Store cassettes in temperature-controlled cases

• Avoid prolonged exposure to freezing conditions

• Allow cassettes to acclimate to indoor temperatures before use

• Follow manufacturer storage guidelines closely

This step alone can make the difference between defensible data and questionable results.

Is Air Sampling Recommended During Snowy Conditions?

This is where the answer becomes nuanced.

The Challenge of the Outdoor Baseline

In mold testing, indoor air samples are typically compared to an outdoor reference sample. In winter:

• Outdoor counts may be artificially low

• Snow-covered ground suppresses spores

• The “baseline” may not reflect true environmental conditions

This can make indoor samples appear elevated, even when no active indoor amplification exists—or conversely, make interpretation difficult due to the lack of a meaningful comparison.

So… Should You Still Air Sample?

Sometimes—yes. But with caution.

Air sampling may still be appropriate when:

• There are clear indoor complaints (odors, symptoms, visible growth)

• The goal is indoor comparison (room-to-room, floor-to-floor)

• Samples are interpreted alongside visual inspection and moisture data

However, air sampling should not be used in isolation during inclement weather.

A Better Winter Testing Approach

During snowy or freezing conditions, mold assessments should emphasize a holistic inspection strategy:

• Visual inspection of known moisture-prone areas

• Moisture mapping (walls, ceilings, basements, crawlspaces)

• Thermal imaging to identify hidden condensation or leaks

• Surface or cavity sampling when justified

• Context-aware interpretation of any air samples collected

In winter, the inspector’s expertise matters more than the numbers alone.

Key Takeaways

• Cold weather suppresses outdoor mold spores, skewing baselines

• Spore trap cassettes can be damaged by freezing temperatures

• Outdoor air samples may not reflect “normal” conditions

• Indoor mold issues still exist year-round

• Winter testing requires judgment, not just data

Final Thought

Mold testing during inclement weather isn’t wrong—but it demands a higher level of care, context, and interpretation. Whether you’re a homeowner or an inspector, understanding these limitations ensures results are accurate, defensible, and meaningful.