AuntieGen Explains: How Contaminants Move Through a Building

When contamination is discovered inside a building, one of the first questions people ask is how far it may have travelled. If mold was found in the basement, could anything from that environment have reached the second floor? If contamination occurred in one room, what connected that room to the rest of the building? If something happened at work, could material have travelled home on clothing, shoes, belongings or even in a vehicle?

These sound like simple questions, but buildings are more complicated than the rooms we see.

We build walls, floors and ceilings to create boundaries. We call one space the basement, another the bedroom, another the attic and another the mechanical room. In an office building, we may think of the third floor as completely separate from the fifth. Those boundaries are useful to us, but air, moisture, pressure and mechanical systems do not necessarily stop at the lines we have drawn.

A building is a connected environmental system. Once contamination is introduced into that system, understanding how it may move means understanding the building itself.

That does not mean contamination found in one location has automatically spread everywhere. Different contaminants behave differently, and every building presents a different set of conditions. Mold spores do not behave exactly like chemical vapors. Methamphetamine residue is not the same as a virus. Biological material from a death scene presents different concerns from off-gassing from new flooring, paint or cabinetry.

Regardless of what we are investigating, however, one question becomes extremely important:

What pathways were available for it to move?

A Building Never Stops Moving Air

Even when the air inside your home or workplace feels perfectly still, it isn’t.

Air moves in response to differences in temperature and pressure. Wind pushes against a building, creating positive pressure on the side it strikes while contributing to lower-pressure areas elsewhere. Air can be pushed into one portion of the structure while being pulled out of another.

Doors and windows open and close. Bathroom exhaust fans, kitchen hoods and clothes dryers remove air. Fireplaces can draw air from inside the structure. Heating and cooling equipment cycles on and off.

Whenever air leaves a building, replacement air has to come from somewhere.

Sometimes it enters through an open door or window. It can also enter through places you never see: gaps around plumbing and electrical penetrations, cracks, rim joists, attic accesses, utility openings, wall cavities and countless imperfections in the building envelope.

The direction of that movement isn’t necessarily constant. Change the outdoor temperature, wind direction, operation of an exhaust fan, position of a door or operation of the HVAC system and you can change pressure relationships inside the building.

In larger structures, even elevator movement, stairwells and repeatedly opening exterior doors can become part of the pressure picture.

Why does any of this matter?

Because air can carry things with it.

Depending on their size, weight and physical characteristics, spores, fragments, dust, aerosols and other particles can become suspended in moving air. Gases and chemical vapors can travel with airflow as well, although they behave differently from particles.

A building doesn’t have to feel drafty for any of this to occur.

Stack Effect: The Building’s Natural Elevator

One of the forces behind vertical air movement is called stack effect.

Warm air is less dense than cooler air and tends to rise. During colder weather, warm indoor air can rise through a building and escape through openings higher in the structure. As that air leaves, the lower portion of the building can become negatively pressurized relative to outdoors, drawing replacement air into lower areas.

That replacement air might enter through intentional ventilation. It can also be drawn through a basement, crawlspace, foundation opening, attached garage or another leakage pathway.

Now consider what connects those lower spaces to everything above them: stairways, wall cavities, plumbing and electrical penetrations, ductwork, utility chases, floor openings and countless small construction gaps.

In taller buildings, stack effect can become much more pronounced because there is a greater vertical distance over which pressure differences can develop. Elevator shafts, stairwells, mechanical chases and utility shafts can become important parts of the airflow picture.

Conditions can also reverse. During hot weather in an air-conditioned building, reverse stack effect can influence airflow in the opposite direction.

Stack effect isn’t something that simply turns on and remains constant. It changes with indoor and outdoor temperatures, building height, weather, season and mechanical operation.

The same building can move air differently in January than it does in July, and differently at two in the afternoon than it does at two in the morning.

HVAC: The Mechanical Highway Through a Building

Then we add the heating, ventilation and air-conditioning system.

An HVAC system intentionally moves large volumes of air. Return systems draw air back toward the equipment. That air may pass through filtration, heating or cooling equipment and then be redistributed through supply ducts to different areas of the building.

If contaminants are present in air entering a return system, understanding where that air goes becomes important.

But this is where HVAC movement becomes more interesting than simply asking what went into a duct.

A contaminant does not necessarily have to enter the ductwork for the HVAC system to influence where it moves.

Supply and return systems create pressure differences throughout a building. A room receiving more supply air than is being returned may become positively pressurized relative to an adjoining space. Another area may become negative. Closing a bedroom or office door can change those relationships. So can a dirty filter, blocked return, leaking duct or improperly balanced system.

Duct leakage adds another layer. A return duct leaking in an attic or crawlspace can draw air from that environment. A leaking supply duct can pressurize the space surrounding it and influence how air moves elsewhere.

Cooling equipment also changes moisture conditions. Air moving across a cold evaporator coil can produce condensation, which must be collected and drained properly. The condition of the coil, drain pan, plenum, filtration and duct system can therefore become part of understanding the indoor environment.

In a commercial building, the picture can become considerably more complicated. Multiple air handlers, rooftop units, shared returns, mechanical rooms, different HVAC zones and changing occupancy can affect hundreds or thousands of people and many different areas of the structure.

The room where contamination was discovered may be only one point within a much larger mechanical system.

Attics, Walls, Basements and Crawlspaces Are Part of the Building Too

Some of the most important pathways in a building exist in places occupants rarely enter.

An attic, for example, is not simply an empty space above the ceiling.

A properly designed attic may intentionally move outdoor air through soffit vents and allow it to exit through ridge, roof or gable ventilation. Wind changes pressure around the roof, and solar heating can make attic temperatures dramatically different from those in the occupied space below.

At the same time, the ceiling separating the house from the attic is rarely perfectly airtight. Wiring penetrations, recessed lights, plumbing openings, attic hatches, duct penetrations and improperly terminated exhaust systems can create connections between the two environments.

Wall cavities provide additional pathways. Plumbing and electrical wiring travel through them. Holes are drilled through framing. Floor joist bays intersect walls. Dropped ceilings can connect multiple rooms. In larger buildings, plumbing chases and mechanical shafts may extend through several floors.

Then there is everything below us.

A hundred-year-old stone or block basement does not behave exactly like a modern poured-concrete foundation. A vented crawlspace is different from an encapsulated and conditioned crawlspace. A slab-on-grade building has no basement at all, but it still contains plumbing penetrations, utility openings, slab cracks and wall-to-floor connections.

Water adds another dimension.

A roof leak may enter in one location and become visible somewhere else. Plumbing leaks can follow framing and building materials. Foundation seepage, groundwater and soil moisture can affect basements and crawlspaces. Water vapor can move without anyone seeing liquid water, and condensation can occur when moisture-containing air reaches a sufficiently cold surface.

For mold, that moisture history becomes particularly important. The place where mold is eventually discovered may not tell the entire story of where the moisture began or what happened before anyone knew there was a problem.

A 100-Year-Old Building and a New Building Behave Differently

Building age changes many of these relationships, but newer does not automatically mean better and older does not automatically mean worse.

Older buildings may contain generations of renovations. Walls have been opened and closed. Plumbing and electrical systems have been added. Heating systems may have been converted several times. Windows have been replaced. Additions may have been attached to original structures. Mechanical systems installed decades later may interact with a building that was never designed for them.

A 40-year-old or 20-year-old building may present an entirely different combination of construction methods, insulation, air sealing, mechanical equipment and renovations.

Modern construction tends to be tighter and more energy efficient. That can reduce uncontrolled air leakage, but it also makes intentional ventilation and mechanical balance increasingly important.

New buildings introduce another environmental consideration: the materials themselves.

Paints, stains, varnishes, adhesives, sealants, flooring, cabinetry, composite wood products and other construction materials can release volatile organic compounds and other chemicals into indoor air. This process is commonly called off-gassing.

A vapor released from a building material behaves differently from a mold spore sitting in settled dust. Temperature, ventilation, airflow and the chemistry of the material can influence how a vapor disperses and how long concentrations remain elevated.

A building doesn’t have to be old, water damaged or visibly dirty to have an indoor environmental concern.

Different Contaminants Move in Different Ways

This is where understanding what we are actually dealing with becomes so important.

We use the word contamination as though it describes one thing.

It doesn’t.

Mold can produce spores and microscopic fragments that may become airborne or settle into dust and onto surfaces. Activity and disturbance can change what becomes airborne and where settled material goes.

Bacteria and viruses vary enormously. Depending on the organism and circumstances, movement or transmission may involve aerosols, droplets, contaminated surfaces, bodily fluids or direct human activity.

Methamphetamine contamination can leave residues on surfaces and contents. Fentanyl and other clandestine drug contamination can involve powders, particulates, surfaces and materials that require very different considerations.

A death scene or other biological event may involve blood, bodily fluids, microorganisms, decomposition products, affected building materials and contents.

Chemical contaminants create another category. Vapors from solvents, paints, stains, varnishes, adhesives and other compounds can disperse through the air without behaving like particulate contamination at all.

Some contamination travels suspended in air. Some settles. Some adheres to surfaces. Some moves as a vapor. Some moves with water. Some is physically carried.

Sometimes more than one mechanism is involved.

That is why simply asking “Can contamination move?” doesn’t tell us very much.

The better question is:

What are we dealing with, and how can this particular contaminant move under these particular conditions?

People, Pets, Belongings and Vehicles Move Things Too

Not all contaminant movement is caused by the building.

Sometimes we move it ourselves.

Imagine walking through an affected area. Your shoes contact the floor. Your clothing brushes against surfaces. You pick up your phone, grab your bag, move a box, carry tools or sit on furniture.

Then you walk somewhere else.

That doesn’t mean every person who enters a contaminated environment automatically carries meaningful levels of contamination somewhere else. It means physical transfer is a pathway that should be considered when the contaminant and circumstances make that possible.

Pets are mobile occupants too. Dogs and cats travel between rooms, floors and outdoors. Their paws and fur can contact dust and surfaces before they move to furniture, bedding, vehicles or another area of the building.

Now imagine having twenty people over for a party.

Doors repeatedly open and close. Twenty sets of shoes, coats and belongings enter the house. People move between the kitchen, bathroom, basement and living areas. Indoor temperature and moisture levels change. The HVAC system responds to a suddenly different load.

Occupancy changes the building.

The same principle applies on a much larger scale to offices, schools, hospitals, restaurants, hotels and retail facilities.

And movement doesn’t necessarily stop at the front door.

Someone leaves a workplace and gets into a vehicle. Shoes contact the floor mat. Clothing contacts the seat. A work bag is placed in the back. Tools or equipment go into the trunk. That person drives home, carries belongings inside and continues with the day.

The vehicle has become another environment along the pathway.

Furniture, mattresses, clothing, cardboard boxes, stored belongings, tools and equipment can also travel from one environment to another. During a move or renovation, contents that have remained in one building for years may suddenly be transported somewhere completely different.

What Happens After Contamination Can Change Where It Goes

There is another part of contaminant movement that often occurs before anyone realizes there is a problem.

Someone cleans.

Someone vacuums or sweeps. A contractor opens a wall. Carpet is pulled up. Furniture is carried through the house. Drywall is demolished. A fan is placed in the room. Windows are opened. The HVAC system continues operating.

None of those actions necessarily means someone did something wrong. Most people act based on what they know at the time.

But disturbing contaminated material can change its physical condition and, depending on what it is, change how it moves.

Renovation can open cavities that had been enclosed for decades. Demolition can generate dust. Moving contents can create physical transfer pathways. Cleaning equipment used in one location and then another can transport material.

This is why the history of a contamination event matters so much.

By the time an investigator arrives, the building may not be behaving exactly as it did when the problem began.

Seasons may have changed. Heating became cooling. Windows were opened and closed. People moved in and out. Pets travelled through the rooms. Furniture was moved. Repairs occurred. HVAC equipment may have cycled thousands of times.

The contamination may have been there long before anyone knew to look for it.

Follow the Pathways, Not Just the Place Where It Was Found

When AuntieGen looks at a contaminated building, the visible problem is important, but it is only one part of understanding what happened.

We want to understand the contaminant itself. We want to understand the HVAC system, airflow and pressure. We look at temperature and humidity, moisture pathways, attics, crawlspaces, basements, foundations and hidden cavities. We consider construction, renovations, weather, occupancy and the movement of people and contents.

And we want to understand time.

Because finding contamination tells us where it was found today.

It doesn’t necessarily tell us where it began, what conditions existed six months ago, what the HVAC system was doing when the weather was different, what was moved before anyone knew there was a problem or what pathways connected that location to the rest of the building.

Buildings have boundaries because we build them.

Air, moisture, pressure, people, animals and mechanical systems create pathways through and around those boundaries every day.

So when contamination is discovered, sometimes the most important question isn’t simply:

Where is it?

It is:

Where did it begin, what pathways were available to it, and what happened inside this building before anyone knew it was there?