Why do my windows look dirty again days after cleaning if I live near a freeway?
Because the deposit is not dust and it was probably washed rather than degreased. Tire wear, brake particulate and diesel soot come off a roadway as very fine carbon bound up in oil. Oil-bound carbon behaves nothing like mineral dust: it darkens glass rather than clouding it, it smears under a dry cloth, and household detergent moves it around the pane rather than lifting it. It also lands several times harder on the elevation facing the corridor than on the sheltered side, which is why one side of a house can look wrong while the rest looks fine. It needs a degreasing pass with real dwell time before the wash, and that elevation honestly needs a shorter service interval than the rest of the building.
When it happens: Continuous and independent of weather, because it follows traffic rather than climate.
Can it be fixed?
Usually removable
It comes off completely with the right chemistry, and the glass underneath is generally undamaged. This is one of the more satisfying problems to diagnose, because homeowners often believe their windows are permanently hazed when they have simply never been degreased. The one genuine risk is scratching: oil-bound carbon holds grit, and dry-wiping it drags that grit across the surface. What will not change is the rate at which it returns. That is a function of where the building sits, and the honest answer is a shorter interval on the affected elevation rather than a better product.
Roadway particulate is not soil. It is dominated by tire tread wear, brake pad abrasion and combustion soot, all of it very fine and much of it carbonaceous.
Vehicle exhaust and road surfaces also emit semi-volatile organic compounds, which condense onto cool surfaces as a thin oily film. That film binds the carbon rather than letting it sit loose.
Because the binder is oil rather than water, plain rinsing does very little, and a water-based detergent at ordinary concentrations emulsifies it poorly. What actually happens under a cloth is that the film redistributes across the pane, which reads to the eye as a smear rather than as removal.
Deposition drops off sharply with distance and with shelter, which is why the effect is so uneven across a single property. The corridor-facing elevation can receive several times what the opposite side does, on a schedule set by traffic rather than by weather.
The degreasing step adds modestly to a normal cleaning rather than doubling it, and it should apply only to the affected elevations rather than the whole property. The real cost conversation is frequency: a corridor-facing elevation may genuinely need three or four services a year while the rest of the house needs two. Any company that prices the whole property at the worst elevation is overcharging you for the sheltered side, and any company that prices it at the best one is going to leave you disappointed.
A degreasing dish soap at a stronger concentration than you would normally use for glass, applied warm and given a few minutes of dwell before agitation, lifts most of it. The order matters more than the product: flood first, let it work, then squeegee, and never dry-wipe the initial pass. Automotive glass cleaners formulated for road film also work well on house glass. What does not work is plain water, ordinary spray glass cleaner, or scrubbing harder; the last of those is how the trapped grit ends up scratching the pane.
Removing it once is half the job. This is the half that determines whether you pay for it again next year.
This problem is not evenly distributed. These are the markets where local conditions make it common, and why.
Directly against the largest port complex in the western hemisphere, so diesel particulate is continuous and independent of weather.
Port, rail and freeway corridors on one side and ocean salt on the other, producing two different deposits on one house.
Sits between the 405 and the 10, so corridor film rather than mineral is the dominant local deposit.
An international airport plus one of the densest warehouse networks in the western United States.
One of the region's larger distribution footprints, with particulate arriving on a traffic schedule rather than a weather one.
The 10, the 101, the 110 and the 5 all run through dense residential fabric, so this affects an enormous number of homes.
Almost certainly not. This is one of the most common false conclusions in window cleaning near a corridor. Oil-bound carbon looks like permanent haze and behaves like permanent haze under ordinary glass cleaner, and then comes off entirely the first time somebody degreases it properly. Test one pane with a strong degreasing soap and real dwell time before assuming the worst.
Because deposition drops off sharply with distance and shelter. The elevation facing the corridor can receive several times what the opposite side does. That is also the useful part: it means the answer is a shorter interval on that elevation specifically rather than more frequent cleaning of the whole property.
It follows traffic rather than weather, so the deposition rate is fairly constant. What changes seasonally is that between May and November there is no rain to remove any of it, so the accumulation is uninterrupted. Then the first storm of the season mobilises it and runs it down the pane in streaks, which is when most people call.
For the inside, yes; for the outside, no. This is entirely an exterior deposition problem. Newer windows will not resist it and interior filtration does not touch it. Landscaping or fencing that breaks the line of sight to the corridor does measurably reduce what lands on ground-floor glass, which is the only genuine mitigation short of moving.
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