Black Particles and the HVAC System

“I have a tenant who has reported black particles shooting out of the ducts,” is a common client concern for us at Healthy Buildings. We have investigated hundreds of complaints regarding black particles and the HVAC system. Here is guidance on addressing complaints of black particles from HVAC systems.

Black Cladosporium species mold growing in an HVAC fan chamber. Notice the small, rounded nodules of mold that grow at night and dislodge when the fan is energized.

Black particles trapped in filter and on top of file cabinets under a supply diffuser indicate the HVAC system as the source.

  • Determine if the HVAC system is really the source. In many cases, the HVAC system is not the source. Occupants may point to a dirty supply diffuser as proof, but staining around the diffuser typically indicates a dust source in the space. For example, it is a high traffic area, near a loading dock, near copiers or printers, etc. The occupant generated particles get suspended in the air, caught by the air exiting supply diffusers, and impacted on ceiling tiles due to turbulent air currents. This type of cosmetic problem can be reduced by proper housekeeping and preventing particles from entering the building by keeping doors and windows closed and indoor areas pressurized to outdoors. Identifying the proximity of the black particles to air outlets can help determine if the HVAC system is the source. Look at elevated horizontal surfaces under the diffusers, such as the top of file cabinets or office cubicles. If the particles are only seen on an occupant’s desk and not above the desk, the occupant might be the source. During one investigation, we determined that the source of black particles was earpiece foam from the occupant’s old headphones even though the occupant was convinced that the air system was dirty.

  • If the HVAC system is the suspected source, inspect it. Black particles from the HVAC system are typically mold, rust or insulation.The air handling units, variable air volume (VAV) boxes, reheats, induction units, heat pumps or secondary units serving the area of concern should be inspected.  Are supply plenums or VAV boxes insulated? Review the maintanence logs. Has there been any cleaning or work on the system that may have damaged insulation or dislodged rust? While only a laboratory analysis can determine if mold is growing in the system, look for signs of heavy rust or deteriorating internal insulation in the air system.

  • Place white polyester media filter over the outlet to trap black particles. A standard ½ inch, polyester roll media filter is generally efficient enough to capture these large, visible particles. If you use white media, you can readily see the particles. If you don’t trap the black particles in the media, the HVAC system is probably not the source. If you do, the media can be analyzed by a laboratory to identify the make-up of the dust collected.

  • Call in an expert. Since black particles and HVAC system issues are generally prompted by occupant health concerns, expert assistance can be beneficial. An experienced investigator can save you time and money. For example, before consultation, a building engineer with black particle problems collected a sample and sent it to the wrong type of lab. A lab with little microbiological experience determined that the particles were foam. For the next year, engineers systematically inspected and dismantled the air system for evidence of foam. It became an expensive and time consuming process. The tenants became frustrated by the lack of resolution to this issue. Healthy Buildings then inspected the system and discovered Cladophialophora, a Cladosporium species of fungi growing in the air systems. This mold would grow over night in the humid fan chamber and, as the systems turned on each morning, the air movement would dislodge small mold particles from the fan chamber walls. These particles blew through the duct system and out the supply registers. Tenants are usually more concerned with mold particles falling on their heads than foam! After Healthy Buildings’ inspection, the issue was properly corrected by cleaning the affected areas and improving humidity control within the air system. The key to solving this issue was hiring an experienced professional who could identify probable locations of particle sources, sample properly and determine the best laboratory for the material sampled. Laboratory particle identification can help determine the particle source and may alleviate tenant health concerns. Sometimes, mold looks like common debris in the air system and the naked eye cannot determine if the black particle is rust or insulation, but identifying particles under a microscope is difficult. Laboratories have difficulties identifying “unknowns” so observations and reference samples are helpful. Visual observations should be logged with samples to help the lab interpret what they see under the microscope. The lab can supplement optical tests with pressure and magnetic tests to determine if the particles are insulation backing or metallic (i.e., rust). If the suspect particles are collected in the tenant space, it is prudent to collect reference samples from suspected sources in the building. The laboratory compares problem particles and suspected particle sources for quick and cheap confirmation of the source.

For further information, or to schedule an Indoor Air Quality inspection, please contact us at

By | 2017-08-08T18:36:06+00:00 August 14th, 2014|Blog, News & Announcements|Comments Off on Black Particles and the HVAC System

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