New York Mold Assessors — Special-Circumstance Mold Investigations

A multi-disk or multi-plate viable air-sampling investigation can be appropriate for certain complex mold projects when the objective is to evaluate culturable airborne microorganisms across multiple locations, conditions, or stages of a remediation project. An Andersen-type viable impactor operates by drawing a measured volume of air through precision openings and impacting airborne particles onto an agar culture medium. The collected plates are then transferred to a qualified laboratory for incubation and analysis. The Andersen N6 single-stage configuration is commonly operated at 28.3 L/min (1 CFM) and has a reported 50% aerodynamic cut point of approximately 0.65 µm.

WHAT DOES “MULTI-DISK” MEAN?

In the context of a mold investigation, multi-disk does not necessarily mean that several disks are placed into the impactor simultaneously. More commonly, it means that multiple culture plates are collected as separate samples at different locations or under different sampling conditions.

For example, a complex project might involve:

Disk/Plate #1 — Outdoor reference
Disk/Plate #2 — Unaffected interior area
Disk/Plate #3 — Mold-affected area
Disk/Plate #4 — HVAC/adjacent area
Disk/Plate #5 — Containment area
Disk/Plate #6 — Area outside containment
Disk/Plate #7 — Post-remediation area

Each plate becomes an individual sample with its own location, identification, sampling information, and laboratory result.

This approach can be especially useful when the investigator needs to understand differences between locations rather than relying on a single air sample.

SPECIAL CIRCUMSTANCES WHERE THIS APPROACH MAY BE USEFUL

A multi-location viable sampling strategy may be considered for projects involving:

• Extensive visible mold-like growth

• Large water-loss events

• Commercial buildings

• Multifamily buildings

• Condominiums and co-ops

• Schools and childcare facilities

• Healthcare or sensitive environments

• HVAC contamination concerns

• Suspected cross-contamination

• Large remediation containment areas

• Multiple affected rooms

• Multiple floors

• Recurring mold after previous remediation

• Disputed remediation results

• Post-remediation verification objectives

• Investigations involving occupied and unoccupied areas

• Projects where comparison between indoor locations and an outdoor reference is useful

The important principle is that the sampling plan should be driven by the investigative question, rather than simply collecting a large number of plates.

WHY AN ANDERSEN IMPACTOR?

The Andersen-type impactor is a viable/culture-based sampler. Air is drawn through the impactor and airborne particles are deposited onto the culture medium. Following collection, the plate is submitted to the laboratory for appropriate incubation, enumeration, and identification procedures.

The N6 single-stage Andersen configuration is designed for estimating the concentration of viable airborne microorganisms, but it does not provide the particle-size distribution information obtained from the complete six-stage Andersen cascade impactor. Research comparing the modified single-stage approach with the traditional Andersen system found it useful for estimating total viable airborne fungal concentration.

AN IMPORTANT DISTINCTION: VIABLE VS. TOTAL AIRBORNE FUNGI

This is one of the most important points for New York Mold Assessors and remediation professionals.

A viable culture sample measures what can grow under the laboratory's selected culture conditions. It does not represent every fungal particle present in the air.

Therefore:

No growth ≠ no fungal material.

Low CFU ≠ automatically clean or safe.

High CFU ≠ automatically a health hazard.

A culture result ≠ proof of occupant illness.

The results need to be interpreted in conjunction with the visual inspection, moisture conditions, building history, sampling locations, HVAC conditions, remediation scope, and other available evidence.

CDC/NIOSH materials specifically note that viable sampling is culture-based and that there are currently no OSHA or ACGIH exposure limits for biological agents applicable as a universal pass/fail threshold for indoor mold investigations.

SPECIAL REMEDIATION SCENARIO

Consider a building that experienced a significant water loss.

The remediation contractor establishes containment around a heavily affected room. The investigator wants to determine whether the environmental conditions differ between the affected space and surrounding areas.

A properly designed project might evaluate:

OUTDOOR REFERENCE → CLEAN INTERIOR → AFFECTED AREA → CONTAINMENT PERIMETER → ADJACENT AREA → POST-REMEDIATION AREA

The value of this approach is not simply the number of plates collected. The value comes from being able to associate each result with a specific location and specific investigative question.

If the project involves containment, the sampling strategy should also account for HVAC operation, pressure relationships, occupant activity, doors, airflow pathways, and the timing of remediation activities, because these factors can influence airborne concentrations.

POST-REMEDIATION APPLICATION

For post-remediation work, viable sampling may be considered when the project specifications call for it. However, laboratory results should not be used as the sole clearance criterion.

A post-remediation assessment should first consider whether:

• The moisture source has been corrected

• Affected materials identified in the scope have been addressed

• Visible mold-like growth has been removed

• Dust and debris have been removed

• Containment has been appropriately dismantled

• HVAC concerns have been addressed

• The area is visually acceptable

• Moisture conditions are acceptable

• Additional testing is warranted

Only then should viable sampling results be interpreted within the context of the entire project.

NEW YORK MOLD ASSESSORS — PROFESSIONAL APPROACH

New York Mold Assessors can structure viable air-sampling projects around the specific question that needs to be answered:

BEFORE REMEDIATION

Establish appropriate baseline information.

DURING INVESTIGATION

Characterize selected areas and potential pathways.

DURING COMPLEX REMEDIATION

Evaluate specifically defined containment or cross-contamination questions where appropriate.

AFTER REMEDIATION

Conduct a post-remediation assessment and, when justified, targeted viable sampling.

DOCUMENT EVERYTHING

Record the sample location, environmental conditions, sampling parameters, chain of custody, laboratory results, photographs, and interpretation.

The objective is to create a defensible environmental record, not simply generate laboratory numbers.

NEW YORK MOLD ASSESSORS

SEE THE CONDITION.
INVESTIGATE THE MOISTURE.
ESTABLISH THE SAMPLING OBJECTIVE.
COLLECT REPRESENTATIVE SAMPLES.
DOCUMENT THE CONDITIONS.
INTERPRET THE LABORATORY RESULTS IN CONTEXT.
VERIFY THE REMEDIATION OBJECTIVES.

A multi-plate viable air investigation is most valuable when every sample has a purpose.

New York Mold Assessors — Assessment identifies the condition. Documentation establishes the facts. Investigation identifies potential contributing factors. Testing provides additional information when appropriate. Remediation addresses the environmental problem. Verification documents the completed work.