If two commercial buildings in Tulsa have their windows professionally cleaned on the same day, there is no guarantee they will look equally clean a month later.
One may still look relatively clear. The other may already show dust, spotting, residue, or a noticeable film.
At first, the difference might appear to come down to the quality of the cleaning. In reality, research into the soiling of exterior glass suggests something more complicated: the rate at which glass becomes dirty is strongly influenced by what happens around the building after the cleaning is finished.
Studies of architectural glass have examined factors including airborne particulate matter, pollution, exposure to precipitation, surrounding environmental conditions, and the physical characteristics of the glass itself. Taken together, that research helps explain why there is no universal amount of time that a professionally cleaned window should remain visibly clean.
For property managers, this distinction matters. Understanding why a building gets dirty can be more useful than simply asking how often it should be cleaned.
A window is constantly exposed to the atmosphere surrounding it.
That atmosphere can contain dust, soil particles, combustion products, pollen, organic material, salts, and other airborne contaminants. Once those materials reach the glass, some remain on the surface and gradually affect its appearance.
Research published in Atmospheric Environment examined the soiling of modern glass under polluted urban conditions and found that particle deposition was strongly related to the distribution and concentration of particulate matter in the surrounding environment. Researchers also observed greater deposition at locations closer to particulate sources, including roadside environments. The study can be reviewed through ScienceDirect here.
That finding has practical implications for commercial properties.
Consider two office buildings that are otherwise nearly identical. One sits several hundred feet away from a heavily traveled roadway and is surrounded primarily by landscaping. The other sits directly beside a major intersection with a large parking lot and nearby construction.
The windows may have the same glass.
They may have been cleaned using the same process.
But they are not operating in the same environment.
The U.S. Environmental Protection Agency identifies roads, agriculture, construction, industrial processes, fires, and mobile sources among contributors to particulate matter emissions in the United States. EPA data also show that road, agricultural, and construction dust represent substantial contributors to larger airborne particulate matter. The EPA's overview of particulate matter sources provides additional information here.
That does not mean every building near a road will immediately become dirty. It does mean that location should be considered when determining how quickly exterior surfaces are likely to accumulate contamination.
Commercial buildings are often surrounded by something residential properties are not: large amounts of vehicle activity.
Cars enter and exit parking lots throughout the day. Delivery trucks move through loading areas. Nearby roads carry continuous traffic.
That activity can contribute to airborne material in several ways.
EPA research has specifically examined road dust generated by vehicle movement, noting that particulate emissions can occur as vehicles travel across paved surfaces containing deposited material. That surface material can come from pavement wear, vehicles, nearby unpaved areas, and other surrounding sources. EPA research on road dust and particulate matter can be found here.
For a commercial property, proximity matters.
A building located immediately beside a busy parking lot may experience different exposure from one positioned farther away from vehicle traffic, even if the two properties are in the same neighborhood.
This is one reason cleaning frequency should not necessarily be determined by building size alone.
The environment immediately surrounding the glass can matter just as much as the amount of glass being cleaned.
The building itself also changes the equation.
Windows can be recessed several inches into a facade, positioned beneath large overhangs, completely exposed to weather, located underneath architectural ledges, or installed on walls facing different directions.
Those differences change what reaches the glass.
Research on glazing exposed to atmospheric pollutants has found that exposure to precipitation and the physical position of glazing can influence how contamination affects the surface. One study involving hundreds of windows and long-term glass exposure found precipitation exposure and glazing characteristics to be important factors affecting the loss of light transmission associated with dirt accumulation. That research is summarized here.
The effect can sometimes be seen on a single building.
One elevation may remain relatively presentable while another becomes visibly dirty much sooner.
That does not necessarily indicate inconsistent cleaning. The two elevations may simply be experiencing different environmental conditions.
A wall facing prevailing weather may receive more wind-driven moisture and airborne material. Another section may sit beneath an overhang that prevents rain from reaching the glass. Lower windows may be exposed to landscaping, pedestrian traffic, irrigation, and parking lots that upper-level glass never encounters.
The building's architecture effectively creates different micro-environments around its windows.
Rain is often assumed to make windows dirty.
Sometimes it contributes to visible spotting, particularly when water interacts with contamination already present on the surface.
But rain can also remove loose material.
This becomes especially clear when looking at research involving glass intentionally sheltered from precipitation.
A study published in Atmospheric Environment exposed glass to urban pollution while protecting it from rain. Researchers observed continued increases in deposited particles and glass soiling over time. The study is available through ScienceDirect.
Another study examining glass exposure across numerous European and Canadian locations investigated relationships between glass soiling, particulate matter, atmospheric pollutants, and meteorological conditions. The researchers found that particulate matter was an important factor in the optical degradation of the glass. The research is indexed by the National Library of Medicine here.
The takeaway is that rainfall should not automatically be treated as the enemy of clean glass.
In some situations, precipitation can carry contamination onto glass or create visible residue. In others, it can remove loose particles.
What was already on the glass, how the water reaches it, and what happens as that water dries all influence the final result.
This creates an interesting contradiction.
It would seem logical that a window protected beneath an awning or deep architectural overhang would stay cleaner because rain cannot reach it.
That is not necessarily the case.
If airborne particles reach the glass but rainfall does not, there may be fewer opportunities for naturally occurring water to remove loose contamination.
The importance of rain exposure can even be seen in specialized self-cleaning glass technology. Pilkington, a major architectural glass manufacturer, produces a self-cleaning glass with a hydrophilic and photocatalytic coating. According to the manufacturer, daylight helps break down organic material while rainfall spreads across the surface and helps wash loosened contamination away. Pilkington explains the technology here.
Interestingly, Pilkington specifically notes that covered areas can limit the effectiveness of this technology because the glass requires adequate daylight and rainfall. Its application guidance discusses this limitation here.
The principle is useful even when discussing ordinary architectural glass.
Protection from weather does not necessarily mean protection from contamination.
The lower levels of a commercial property face another source of contamination: landscaping.
Sprinklers positioned too close to a building can repeatedly spray exterior glass. Once that water evaporates, dissolved minerals can remain behind.
The U.S. Geological Survey explains that water hardness is primarily associated with dissolved calcium and magnesium. Water can pick up these minerals as it moves through soil and rock, and hard water is well known for producing mineral scale and deposits. USGS provides an overview of water hardness here.
On exterior glass, repeated exposure to mineral-bearing water can create visible spotting that is fundamentally different from ordinary dirt.
We have previously discussed this issue in more detail in our article, "Can You Actually Get Rid of Hard Water Stains, or Is the Glass Ruined?", including why prolonged mineral accumulation can eventually require more than routine window cleaning.
This creates another situation where two sides of the same building can behave differently.
Upper-level windows might primarily encounter airborne contamination and weather.
Ground-floor windows may experience those same conditions plus irrigation overspray, landscaping debris, pedestrian activity, and parking lot exposure.
Naturally, their maintenance requirements may not be identical.
Another factor that property managers sometimes overlook is that a building's environment is not permanent.
A property that normally stays presentable for several months may suddenly begin accumulating visible contamination much faster because a construction project begins nearby.
Construction activity is a recognized contributor to airborne particulate matter. EPA emissions data specifically include construction dust among sources of primary particulate matter. EPA's national particulate matter report provides additional data on these sources.
This means an established window cleaning schedule may need to change temporarily.
A property that performs well on quarterly cleaning under normal conditions might require additional attention during a major neighboring construction project. Once construction ends, the original maintenance interval may again become appropriate.
The important point is that cleaning frequency should respond to conditions rather than being treated as an unchangeable number.
Imagine two identical buildings constructed from the same plans.
Building A sits farther from the roadway, has limited surrounding construction, properly directed irrigation, and landscaping that creates relatively little exposure near the windows.
Building B sits beside a high-traffic intersection, has a busy surface parking lot, sprinklers regularly reaching the lower glass, and an active construction project next door.
Both buildings could receive the same quality of professional window cleaning.
Building B may still appear dirty sooner.
That is not necessarily a failure of the cleaning process. It is the predictable result of exposing a clean surface to a more aggressive environment.
Research into architectural glass soiling supports the broader idea that accumulation is affected by particulate concentrations, pollution sources, meteorological conditions, and exposure characteristics rather than time alone.
A cleaning resets the condition of the glass. It does not control what happens to the glass afterward.
Some buildings stay clean longer because they exist in environments that allow them to.
Others are constantly exposed to traffic, dust, construction, irrigation, landscaping, pollution, or architectural conditions that encourage faster accumulation.
That is why comparing one property's cleaning schedule directly with another can be misleading.
The better comparison is the building against itself.
Observe how quickly different elevations become dirty. Identify recurring sources of contamination. Pay attention to seasonal or environmental changes. Then build the maintenance schedule around those patterns.
A professional window cleaning can establish a clean baseline.
How long that baseline lasts is ultimately determined by everything the building encounters next.
Treshaun Williams is the Content Manager at Squeegee Squad Tulsa, where he documents the people, processes, equipment, and planning behind professional commercial and residential window cleaning throughout the Tulsa area.
Treshaun's work focuses on researching and communicating the factors that property owners and managers may not immediately see, including building access, environmental exposure, glass condition, maintenance frequency, safety considerations, and the planning required before a window cleaning crew arrives on site.
This article draws on published research, government resources, manufacturer documentation, and observations from the window cleaning industry to help property owners better understand the factors that can influence exterior glass maintenance.
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