Black Particles and the HVAC System

Black particles coming out of the ducts is a common customer concern. UL Solutions has investigated hundreds of complaints regarding black particles and the HVAC system. Here are some lessons learned on addressing complaints.

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 may be in a high traffic area, near a loading dock or near copiers or printers. 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. You should inspect air handling units, variable air volume (VAV) boxes, reheats, induction units and heat pumps, or secondary units serving the area of concern. Are supply plenums or VAV boxes insulated? Review the maintenance 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, you can 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 0.5 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. Expert assistance is necessary to address occupant health concerns caused by black particles and HVAC system issues. 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 laboratory. A laboratory 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. UL Solutions then inspected the system and discovered cladophialophora, a cladosporium species of fungi growing in the air systems. This mold would grow overnight 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. 

After UL Solutions’ 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 experienced professionals 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. Further, identifying unknown particles under a microscope can be difficult so observations and reference samples are helpful. Visual observations should be logged with samples to help the laboratory interpret what they see under the microscope. The laboratory can supplement optical tests with pressure and magnetic tests to determine if the particles are insulation backing or metallic (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 confirmation of the source.