The air outside looks hazy, an unusual smell has developed inside your building, and occupants are starting to ask questions. Is the outdoor air affecting the building? Is the ventilation system operating properly? Could moisture, mould, chemicals or another indoor source be contributing to the concern?

An unusual odour or air quality complaint does not always have an obvious explanation. Sometimes the source is temporary and easily corrected. In other situations, it may be a sign that the building requires a more detailed investigation. Understanding what is happening inside the building is the first step toward determining an appropriate response.

Can Outdoor Air Quality Affect Indoor Air?

People often assume that closing the doors and windows completely separates indoor air from outdoor conditions. Buildings, however, are not airtight.

Outdoor air enters through ventilation systems, open doors and windows, gaps in the building envelope and other openings. Fine particles, smoke and odours may therefore enter a building when outdoor air quality is poor.

The degree of impact can depend on several factors, including:

• The condition of the building envelope
• How often exterior doors are opened
• The location of outdoor-air intakes
• The design and operation of the HVAC system
• The type and condition of the filtration system
• Whether the building is operating with positive or negative air pressure
• The length and severity of the outdoor air quality event

Health Canada notes that fine particles and other outdoor pollutants can enter buildings through ventilation systems, doors, windows and gaps in the building envelope. Its guidance on fine particulate matter (PM2.5) in residential indoor air sets out why these particles matter and how indoor levels can be reduced.

Indoor conditions may also remain affected after the outdoor air has improved, particularly when particles have accumulated on filters, furnishings and other interior surfaces.

Monitoring both indoor and outdoor conditions can help determine whether an indoor concern is connected to the surrounding environment or whether another source may be present within the building.

An Odour Is a Clue — Not a Diagnosis

An unusual smell can be an important sign that something has changed, but odour alone cannot identify a contaminant or determine whether the air is safe.

Some substances can be detected by smell at very low concentrations. Other potentially serious contaminants, including carbon monoxide and radon, cannot be smelled at all.

The way people perceive an odour can also vary considerably. One person may notice it immediately, while another person in the same space may not detect it.

Descriptions such as musty, chemical, smoky, earthy, sewer-like or burning can provide useful information during an investigation, but they should be treated as observations rather than conclusions.

Before selecting a testing method, it is important to consider:

• When the odour first appeared
• Where it is strongest
• Whether it changes throughout the day
• Whether it is connected to weather or wind direction
• Whether it appears when HVAC equipment starts
• Whether renovations, cleaning or maintenance recently occurred
• Whether there has been a leak, flood, fire or sewage event
• Whether the concern affects one room or the entire building

These details can help narrow the investigation and identify appropriate next steps.

What Can Cause Indoor Air Quality Complaints?

Indoor air quality concerns can have many possible sources. In some situations, several factors may be contributing at the same time.

Outdoor Smoke and Particulate Matter

Wildfire smoke, nearby fires, construction dust, vehicle exhaust and industrial emissions may affect indoor conditions when contaminants enter through doors, windows or ventilation systems.

Fine particulate matter may be especially difficult to see. Indoor particulate levels can also be influenced by cleaning activities, construction work, equipment and normal occupant activity. Where the source of airborne particles needs to be established, dust identification can help distinguish outdoor infiltration from an indoor source.

Moisture and Mould

Roof leaks, plumbing failures, foundation seepage, condensation and previous flooding can create damp conditions that support mould growth.

A musty or earthy odour may indicate a moisture concern, but the source may be hidden behind walls, beneath flooring, above ceilings or within mechanical systems.

A proper mould assessment should look beyond the odour and investigate possible moisture sources and affected materials. Health Canada’s guidance on addressing moisture and mould emphasizes correcting the moisture source rather than treating visible growth alone. For buildings that have experienced flooding, our article on why competent water damage inspections matter after flooding and CAT events covers this in more detail.

Ventilation Problems

Stale or uncomfortable air does not necessarily mean that a toxic contaminant is present.

The issue may involve insufficient outdoor air, uneven air distribution, blocked vents, dirty filters, poorly maintained equipment or an HVAC system that is not operating as intended.

Temperature, humidity and carbon dioxide measurements can provide useful information about building conditions and ventilation performance. Health Canada publishes a residential indoor air quality guideline for carbon dioxide that is often used as a reference point when evaluating ventilation adequacy.

Cleaning Products and Building Materials

Paints, adhesives, flooring, furniture, disinfectants and other products can release volatile organic compounds and noticeable odours.

This may occur after renovation, maintenance, cleaning or the installation of new materials.

The timing of the complaint and the products recently used in the building can help determine whether targeted testing, an odour assessment or additional ventilation should be considered.

Sewer and Drainage Sources

Dry drain traps, damaged plumbing, floor drains and sewer connections may produce persistent or intermittent odours.

These concerns can sometimes be mistaken for mould or chemical contamination. Identifying where and when the odour occurs is an important part of separating possible plumbing issues from other indoor air quality concerns. Where contaminated water or sewage has entered the building, bacteria sampling may be appropriate.

Combustion Sources

Furnaces, water heaters, fireplaces, generators, vehicles and other fuel-burning equipment can produce combustion-related contaminants when equipment is malfunctioning, improperly vented or used incorrectly. Where a fire has occurred, post-fire consulting can determine whether smoke, soot and combustion residues remain present.

Carbon monoxide cannot be identified by smell. Buildings containing fuel-burning equipment should have appropriate carbon monoxide alarms that are properly installed and maintained, in accordance with Ontario’s carbon monoxide safety requirements.

If a carbon monoxide alarm sounds, occupants should leave the affected area and follow the appropriate emergency instructions.

When Should an Indoor Air Quality Concern Be Investigated?

Not every temporary odour requires an extensive sampling program.

However, a professional investigation should be considered when:

• An odour is persistent, recurring or becoming stronger
• Multiple occupants report similar concerns
• The issue appears in a particular room or area
• The concern began after a leak, flood, fire or sewage loss
• Visible staining, condensation or suspected mould is present
• The HVAC system appears to spread the odour
• Outdoor smoke or pollution may be entering the building
• Symptoms or discomfort appear to improve after leaving the building
• Renovation, demolition or maintenance recently occurred
• Previous attempts to correct the issue have not been successful
• The building owner or manager requires documented findings

If occupants are experiencing an immediate medical emergency, visible smoke is present or a carbon monoxide alarm has activated, the situation should be treated as an emergency rather than waiting for routine environmental testing.

What Happens During an Indoor Air Quality Investigation?

A useful investigation begins with understanding the building and the concern — not by automatically collecting every type of sample available. The appropriate process depends on the building, the complaint history and the suspected source.

An indoor air quality investigation may include the following steps.

Reviewing the Concern

The consultant may speak with the property owner, facility manager or occupants to understand when the concern began, where it occurs and whether it follows a noticeable pattern.

Information about renovations, leaks, cleaning, HVAC maintenance and nearby outdoor events can also be important.

Inspecting the Building

A visual inspection can identify signs of moisture, damaged materials, blocked vents, unusual dust accumulation, potential contaminant sources and other conditions that require further examination.

Mechanical rooms, ceiling spaces, basements, crawlspaces and other less-visible areas may need to be considered depending on the complaint.

Reviewing Environmental Conditions

Measurements may be collected for factors such as:

• Temperature
• Relative humidity
• Carbon dioxide
• Carbon monoxide
• Fine particulate matter
• Total volatile organic compounds
• Formaldehyde
• Combustible gases
• Hydrogen sulfide

The parameters selected should be based on the circumstances. Measuring unrelated contaminants without a clear reason may create data that is difficult to interpret and does not resolve the original concern.

Conducting Targeted Sampling

Where appropriate, the investigation may include mould air, surface or bulk sampling, dust identification, airborne exposure level testing or other specialized testing.

Samples can be submitted to an accredited laboratory for analysis when laboratory confirmation is required.

Comparing Indoor and Outdoor Conditions

When outdoor smoke, dust or pollution is suspected, simultaneous indoor and outdoor measurements may help determine how effectively the building is limiting contaminant infiltration.

This information can also assist facility managers and HVAC professionals when evaluating filtration, outdoor-air intake and building-pressure strategies.

Preparing the Report

The final report should clearly document:

• Areas inspected
• Observations and measurements
• Sampling methods
• Laboratory findings, where applicable
• Potential sources or contributing conditions
• Limitations of the investigation
• Practical recommendations and next steps

The purpose is not simply to produce numbers. The findings must be interpreted within the context of the building and the original concern.

What Can Building Owners Do During Poor Outdoor Air Conditions?

The appropriate response depends on the building and the outdoor conditions, but general considerations may include:

• Monitoring the local Air Quality Health Index and official alerts
• Keeping windows and exterior doors closed when appropriate
• Reviewing HVAC operating settings with qualified personnel
• Using the highest-quality filters the system can safely accommodate
• Ensuring filters are properly installed and replaced when required
• Considering appropriately sized portable air cleaners
• Limiting activities that create additional indoor particles
• Monitoring indoor particulate levels where concerns persist
• Bringing in fresh outdoor air when conditions improve

HVAC changes should be made with an understanding of the system. Installing a filter that creates excessive resistance, or changing outdoor-air settings without considering occupancy and building pressure, can create additional problems.

Don’t Guess About the Air Inside Your Building

An unusual odour may be connected to outdoor smoke, a ventilation issue, hidden moisture, a building material, a cleaning product or another source entirely. The presence or absence of a smell does not provide enough information to determine indoor air quality.

A structured investigation can help property owners, facility managers, employers and occupants understand what conditions are present, identify possible sources and determine an appropriate path forward.

The earlier a recurring concern is investigated, the easier it may be to address before it becomes more disruptive. An odour tells you something has changed. It does not tell you what.

Need an Indoor Air Quality Investigation in Ontario?

IES Consulting provides indoor air quality investigations, odour assessments, mould assessments and environmental testing for residential, commercial, institutional and industrial properties throughout Southern Ontario.

Our service area includes Windsor-Essex, Chatham-Kent, Sarnia-Lambton, London-Middlesex and surrounding communities.

Our consultants evaluate the circumstances, select appropriate testing methods and provide clear reporting with practical recommendations.

If you have noticed an unusual odour, are receiving air quality complaints or are concerned that outdoor conditions may be affecting your building, contact IES Consulting to discuss an appropriate next step.

IES Consulting Head Office 21027 Charing Cross Rd. P.O. Box 50 Charing Cross, ON N0P 1G0

E-mail: info@iesconsulting.ca Phone: 519.256.8388