Why does soot collect on ceilings in spots above incandescent bulbs? Why should a carbon monoxide detector be installed on or near the basement ceiling? Why can a moldy basement or crawl space affect the air quality in the rest of the house?
To answer these questions, you have to know something about how fluids behave and about density.
How Fluids Behave
Air, like water, is a fluid that flows.
We instinctively know this to be true outdoors, because we can see breezes rustle leaves and feel the wind against our bodies. Indoors, air also flows, though not as obviously. Both indoors and outdoors, air is in constant motion and carries gases and particulates as it moves. This is why if someone is brewing coffee in the kitchen on the first floor you can smell the aroma on the second or even third floor. Odors are vapors and gases, and in this case, airflow is carrying the odor of the coffee upward. If bacon is frying in a pan on the stove, and the fat is spattering or smoking, airflow will also carry microscopic droplets of fat that have the bacon aroma.
What is Density?
The mass of something depends on how much matter it contains. Weight is a measure of the relationship between mass and gravity. On the surface of the Earth, someone might weigh 150 pounds, but without gravity in space, the same person would be weightless, even though he or she has the same mass.
Density is a measure of the mass that a given volume of matter contains. The density of water in a cup will be same as the density of water in a bathtub, assuming the temperature of the water in both the cup and the bathroom is the same, because density is an intrinsic property of a substance.
Why would temperature make a difference? Heating a fluid like water causes the molecules in the fluid to move faster and further apart. Then the same amount of matter takes up more space, and the density of the matter is decreased. When water or air is cooled, the molecules move more slowly and move closer together. Then the same amount of matter takes up less space, so its density increases.
Substances that are less dense float on substances that are denser. Sewer gas lacks oxygen and is a mixture of carbon dioxide, water vapor and other gases. This mixture is denser than air. If a manhole is filled with sewer gas, the breathable air will float on top of the sewer gas. That's why utility workers blow fresh air into a manhole that is filled with sewer gas; otherwise, as the workers descend to the lower level of the manhole, they could suffocate due to lack of oxygen.
Natural gas and propane are both used as fuels. Understanding density can help you deal with leaks of these potentially explosive substances. Natural gas is less dense than air, so small leaks of natural gas will rise up, mix with the air and dissipate. Propane, on the other hand, is denser than air, so propane gas can accumulate under a small leak and cause a significant burst of flames if ignited. This is why it's dangerous to light a propane grill after the propane has been on for a few moments. Gasoline fumes are also denser than air, which is one reason why you should never throw a lit cigarette to the ground when filling up your gas tank.
Iron is denser than water, so it sinks. Warm water floats on cooler water. Hot air balloons rise because the hot air within the balloons is less dense than the colder, surrounding air. Even in a house, warm air rises and cool air sinks. Warm air rises up and leaks out of the upper levels through construction and window gaps, and air leaks in through similar gaps at the bottom levels to replace the air that has leaked out.
Who Cares?
If you worry that the air you are breathing indoors might be contaminated with irritants or allergens, you should care, because density is one of the guiding principles of indoor air quality.
If the air rising up from a basement or crawl space contains mold spores or musty odors, those contaminants will spread to above-grade (above ground-level) rooms in the house, especially if the basement air is heated by a boiler, furnace or hot-water heater.
Soot consists of black particles from burning fuel. The particles are extremely small and stick readily to surfaces as soon as they collide. Deposits of soot particles are darkest where there is more airflow, such as above light fixtures where the air is heated, and rises more quickly than air further away from the bulbs. If a boiler in a basement is producing soot due to incomplete combustion, and the soot leaks into the basement air, soot particles will be carried on airflow to rooms above. Soot, which is also produced by jar candles, not only stains walls and ceilings; it is also unhealthy to breathe.
In single family homes, most furnaces, boilers and hot-water heaters are located in basements. Such equipment can emit carbon monoxide, among other combustion gases. If warm combustion gases from this equipment flow into the basement air instead of into the chimney, the gases rise up to the basement ceiling because they are less dense than the basement air. The gases then move along the ceiling until they reach the basement stairs, where they rise up to the first floor. This is why it's a good idea to place a carbon monoxide (CO) detector at or near the basement ceiling, just inside the door leading from the first floor to the basement stairs.
If you live in an apartment, and gas or oil heating equipment is located in a closet within your unit, put one CO detector inside the closet near the ceiling, and a second CO detector elsewhere in the apartment. (Some CO detectors are battery operated, and others are connected to a central alarm system.)
Solvents are volatile organic compounds or VOCs. Latex paints contain mostly water, but "oil" paints contain organic solvents (solvents that contain carbon). Liquid solvents evaporate into vapors, which are denser than air and thus sink to the lower levels of a space. If someone is applying solvent-based paint in a small, closed space in which there is little or no ventilation, the person could breathe in high concentrations of solvents. This situation is potentially fatal.
In "Multi-Organ Toxicity and Death Following Acute Unintentional Inhalation of Paint Thinner Fumes," SA Zaidi et al. describe a case in which an adult male "inhaled an excessive amount of paint thinner vapors" and died 11 days later. "Accidental death due to unintentional inhalation of solvent vapors can also occur suddenly," Zaidi cautions. "This case strongly suggests the need to employ suitable precautionary measures while handling organic solvents in a confined area."
Whenever someone is painting in an enclosed space, whether small or large, there should be plenty of ventilation, particularly if the paint or finish being applied contains solvents.
Sources:
- Zaida SA et al., "Multi-Organ Toxicity and Death Following Unintentional Inhalation of Paint Thinner Fumes," Gujarat, India: National Institute of Occupational Health
Resources:
- "Density of Air," Wikipedia
- Haby, Jeff, "Density of Air," The Weather Prediction
- May, Connie L., "It's Not Always Black Mold," Suite 101
- May, Jeffrey C., "Indoor Chemical Exposures," Suite 101
- "VOC Calculators," Painting and Coatings Resource Center
Disclaimer:The information contained in this article is for educational purposes only and should not be used for diagnosis or treatment without the opinion of a health professional. Any reader who is worried about his or her health should contact a doctor for advice.
Jeffrey May - Jeffrey C. May is a Certified Indoor Air Quality Professional and is principal scientist of May Indoor Air Investigations in Tyngsborough, ...
