It's been hot. Summer hot. So, our window is usually open all day and night. I dust (almost) everyday to keep the electronic dust down. We're all packed in pretty close here, so there's a lot of it. But, during the "spring" months, there is another, more sinister source of mess. Yellow Sand. It comes right in and, like any unsavory visitor, is difficult to get rid of. We have no vacuum.
This is the surface of our table. Admittedly, I often neglect it, since it's in our "laundry room." But, it did get dusted once our twice this week, so it hasn't had all that long to accumulate sand.
You can see that it actually is yellow/gray in color. As to why it is this color, here is what Wikipedia says about its pollutant content (I left the hyperlinks in for anyone curious):
Sulfur (an acid rain component), soot, ash, carbon monoxide, and other toxic pollutants including heavy metals (such as mercury, cadmium, chromium, arsenic, lead, zinc, copper) and other carcinogens, often accompany the dust storms, as well as viruses, bacteria, fungi, pesticides, antibiotics, asbestos, herbicides, plastic ingredients, combustion products as well as hormone mimicking phthalates. Though scientists have known that intercontinental dust plumes can ferry bacteria and viruses, "most people had assumed that the [sun's] ultraviolet light would sterilize these clouds," says microbiologist Dale W. Griffin, also with the USGS in St. Petersburg. "We now find that isn't true."