Rooftop Unit Contamination in an Occupied Medical Office
A 12,000 sq ft, two-story medical office building is served by three 5-ton rooftop packaged units. For six weeks, second-floor staff report a musty odor, headaches, and eye irritation that resolve on weekends and after they leave for the day. The building engineer finds standing water in the drain pan of RTU-2 and a dark biofilm ring; the fiberglass duct liner on the supply plenum downstream of the evaporator coil is damp and has a 3 ft by 2 ft area of visible growth. Four flex duct runs feed four exam rooms. A property manager proposes an antimicrobial fog into the liner and putting the units back in service the same day, because the clinic has a full schedule. A single spore trap from one exam room reports 2,100 counts/m3 Cladosporium and 1,400 Penicillium/Aspergillus-type; the outdoor sample reports 3,500 Cladosporium and 700 Penicillium/Aspergillus-type. Clinic management asks, in writing, whether the building is safe to occupy today.
1. Interpret the sampling results. Would you base a shutdown decision on them? Explain.
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There is no regulatory or universally accepted numerical limit for airborne mold, and a single short-term spore trap cannot define a building. What the numbers do say:
- Cladosporium indoors (2,100) is below outdoors (3,500), which is the usual signature of outdoor air infiltration, not an indoor source.
- Penicillium/Aspergillus-type indoors (1,400) is twice the outdoor level (700). That group is the classic indoor amplification indicator, so it is a flag worth acting on, but one sample from one room, with no recorded pump volume, sampling height, or HVAC operating condition, is far too thin a basis for a verdict.
- Spore trap counts are estimates subject to counting statistics; differences of a thousand counts per cubic meter between two traces can be analytical noise.
The shutdown decision should rest on the walk-through, not the numbers: standing water in a drain pan is a reservoir, and visible growth on liner inside the supply air path means the system is distributing spores and fragments into occupied rooms. A contaminated HVAC system should be taken out of service, isolated, and remediated before it is run again. Until then, provide temporary filtration and negative pressure to the affected zone and keep the unit off. State that the sample was a screening sample taken during an active condition, not a clearance sample.
2. Define the remediation scope for the plenum liner, coil, drain pan, and flex duct. What role, if any, does antimicrobial fogging have?
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Liner: fiberglass duct liner is porous and cannot be cleaned to a Condition 1 state once it has supported growth. Remove and replace the contaminated section, and plan on the full plenum; growth usually extends beyond the visible patch into the seams and cuts. Do the removal under sealed containment with HEPA-filtered negative pressure exhausting away from occupied intakes.
Coil: clean with a coil cleaner compatible with the coil metallurgy, rinse, and HEPA vacuum the fins; confirm the coil is not bypassing air and that the drain pan slopes to the drain.
Drain pan: remove standing water, clean the biofilm, verify pitch, restore the trap and drain line, add a float switch or overflow protection, and correct the cause of the standing water (plugged drain, negative static pressure pulling condensate, or a coil that is freezing).
Flex duct: if the interior is contaminated or wet, replace the run. Flex duct cannot be cleaned reliably; the inner liner is fragile and the corrugations trap debris.
Fogging: a fog is not a substitute for removal. It does not penetrate biofilm, it leaves residue on the coil and downstream surfaces, and it can be distributed through the building. An EPA-registered product labeled for HVAC surfaces may be applied as a directed treatment after cleaning, with the unit off and occupants out, strictly per the label. Then run the system normally and do post-remediation verification.
3. Management asks for a written statement that the building is safe to occupy. What do you provide, and what must you not say?
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You can provide a factual condition report. You cannot provide a medical safety determination.
Provide in writing:
- The observed conditions in standard terms: visible growth and standing water in RTU-2, contamination of the supply-side liner, and the affected zones.
- The recommended scope and the criteria for returning to normal service: source corrected, contamination removed, all surfaces clean and dry, HVAC reassembled with new filters, and post-remediation verification performed by an independent party with both visual inspection and, if used, sampling.
- The interim measures: unit off, zone isolated, temporary HEPA filtration, and a plan for the four exam rooms.
- A note that the earlier single sample was taken while contamination was active and is not a clearance result.
Do not say the building is safe, that the samples prove anyone was made ill, or that the numbers are within a limit. Do not diagnose the staff's symptoms. Recommend they consult their occupational medicine provider or infection-control lead and consider relocating immunocompromised patients and staff until verification is complete. Keep the tone factual; you are describing a building condition, not a health outcome.