Case Study: Tackling Mould Contamination After an HVAC Leak

When it comes to indoor environmental hygiene, mould removal is one of the most complex challenges property owners and contractors face. A single water leak can escalate into a serious contamination issue.

This was the situation for an air conditioning equipment company in Tohoku, Japan. A defect during installation caused water leakage in a newly built home. Mould soon spread beneath the flooring, leading to resident complaints and the urgent need for professional remediation.

The company turned to HYSIA Japan for an objective contamination survey and safe, science-based disinfection treatment.

The Challenge

This case wasn’t simply about cleaning up mould. It required accuracy, fairness, and strict safety management:

• Objective data were critical, since mould data would guide compensation discussions between the contractor and residents.
  
• Complex site conditions made the work more challenging, with extensive HVAC systems, wiring, and piping installed under the floor in the house.
  
• Safety management was essential, as the work had to be carried out in tight spaces without risk to workers or damage to the equipment.
• Temporary Relocation: Residents were required to stay outside overnight while work was carried out.

Our Methodology

1. Initial Mould Survey & Inspection

On Day 1, our team conducted:

• Visual inspection of contamination.
• Airborne and surface-attached mould sampling at 20 locations.
• Resident interviews to clarify equipment use and leak timeline.

Sampling of surface-attached bacteria

Timeline

2. Underfloor Disinfectant Treatment

On Day 2, HYSIA applied an ethanol-based disinfectant with antibacterial properties, proven effective against mould in internal testing. The entire underfloor area was treated while ventilated, then dried quickly using the home’s underfloor HVAC system. 

Given the large space and numerous safety precautions, this step required a full day of careful work.

Treatment with disinfectant

3. Ozone Gas Fumigation

On Days 3–4, the team performed whole-building ozone gas fumigation:

• Ozone penetrated into grooves, gaps, and areas unreachable by liquid disinfectants.
• The treatment neutralized airborne mould, surface-attached mould, and musty odors.
• Residents were required to vacate overnight for safety.

Ozone levels were carefully monitored with specialized equipment to maintain sterilization efficacy while ensuring safe re-entry after treatment.

Ozone Gas Fumigation

4. Post-Treatment Mould Re-Inspection

On Day 4, the same 20 survey points were re-tested. After incubation, the results showed a dramatic reduction in mould counts, to levels equal to or below outdoor winter air samples, and well under the average for a typical residence.

The Results

• Air quality restored: Contamination levels reduced to safe standards.
• Odors eliminated: The musty smell disappeared completely.
• Residents reassured: Confidence was rebuilt with transparent data and visible results.
• Client supported: The HVAC company could demonstrate responsible action and regain trust.

HYSIA’s Perspective

Mould is naturally present in our surroundings, but uncontrolled growth indoors becomes a serious health hazard. 

This project highlighted two key points:

1. Meticulous safety management is vital when working in confined, equipment-heavy environments.
2. Scientific evidence and transparent data build fairness, trust, and effective resolution in contractor-resident discussions.

Services Recap:

• Full mould survey
• Underfloor disinfectant treatment
• Whole-building ozone fumigation
• Post-treatment re-inspection

Mould is naturally present in our living spaces, but excessive indoor contamination can pose serious health risks. Preventing and controlling mould growth in places where people live and work is essential for long-term well-being.

At HYSIA, we align every project with our vision to be the catalyst for a healthier environment, and our mission to create spaces where people can live and work with confidence. We remain committed to delivering high-quality environmental hygiene solutions that safeguard both people and projects.

Contact HYSIA today for expert mould remediation and environmental hygiene solutions.