Indoor Air Quality

Indoor air quality has many components, some of which are physical, others chemical, and a few biological. The most significant of these are briefly described below.

The most important physical aspects of indoor air quality are air temperature and humidity. Air temperatures that are too warm or cool for human comfort can be caused by improper placement or adjustment of thermostats, and by an inability of the heating or air-conditioning system to compensate for extremes of outdoor weather, or to adequately deal with heat generated by machinery or large numbers of people. Excessive or insufficient humidity can be caused by similar problems, including poorly operating or non-existent humidity-control mechanisms within the ventilation system.

Carbon dioxide (CO₂) is a normal constituent of the ambient atmosphere, occurring in a concentration of about 350 parts per million (ppm, on a volumetric basis). However, there are many sources of emission of carbon dioxide inside of buildings, including potted plants and their soil, respiration by humans, and stoves or space heaters fueled by kerosene, propane, or methane. Consequently, the concentrations of carbon dioxide are typically relatively large inside of buildings, especially in inadequately ventilated rooms that are crowded with people. Commonly measured concentrations of this gas are about 600-800 ppm, but in some situations concentrations of thousands of ppm can be achieved. Longer-term exposure to concentrations of carbon dioxide greater than about 5,000 ppm is not recommended. Symptoms of excessive exposure to carbon dioxide include drowsiness, dizziness, headaches, and shortage of breath.

Carbon monoxide (CO) is a product of the incomplete oxidation of organic fuels. Indoor emissions are mostly associated with stoves or space heaters fueled by kerosene or natural gas, with cigarette smoke, or with poorly vented emissions from automobiles in garages or loading docks. Longer-term exposures to carbon monoxide concentrations greater than nine ppm should be avoided, as should shorter-term (about one-hour) exposures greater than 35 ppm. Carbon monoxide is a relatively toxic gas because it combines strongly with the hemoglobin of blood, thereby restricting the ability of the circulation system to transport an adequate supply of oxygen to the various parts of the body. Excessive exposures to carbon monoxide under poorly ventilated conditions can cause headaches, drowsiness, nausea, fatigue, impaired judgement, and other symptoms of insufficient oxygen supply. Anoxia and death can ultimately be caused.

Formaldehyde is a pungent, organic vapor that can be detected by smell at a concentration greater than about 0.2 ppm. There are diverse sources of emission of formaldehyde, including poorly sealed plywoods and particle boards, urea-formaldehyde foam insulation, and many fabrics, carpets, glues, and copy papers. Some people are quite sensitive to formaldehyde, developing symptoms that can include a dry or sore throat, headaches, fatigue, nausea, and stinging sensations in the eyes. Most people can tolerate formaldehyde concentrations of less than 0.5 ppm without developing these sorts of symptoms, but other, hypersensitive people may be adversely affected at concentrations as small as 0.01 ppm. In general, exposures to formaldehyde exposure in work areas should be less than 0.1 ppm.

Volatile organic compounds (VOCs) are a wide range of molecular species that vaporize at normally encountered temperatures. Common examples of volatile organic compounds found in buildings include (in alphabetical order): acetone, butyl acetate, dichlorobenzene, dichloromethane, hexane, octane, toluene, trichloroethane, and xylene. These organic chemicals have diverse sources, including synthetic materials used to manufacture carpets and fabrics, paints, solvents, adhesives, cleaning solutions, perfumes, hair sprays, and cigarette smoke. All of the common VOCs and many others have recommended indoor-exposure limits, which vary depending on the toxicity of the particular chemical, and on the length of the exposure. Human responses to large concentrations of volatile organic compounds include dizziness, fatigue, drowsiness, tightness of the chest, numbness or tingling of the extremities, and skin and eye irritation. Some people are hypersensitive to specific compounds or groups of VOCs.

The gases nitric oxide (NO), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂) may also be important pollutants of the indoor atmosphere, especially where there are fuel-burning appliances or stoves used for cooking or space heating. These gases can be irritating to the eyes and upper respiratory system of people exposed to large concentrations.

Radon is a radioactive gas emitted by a wide range of geological sources, including mineral-containing building materials and ground water. Many poorly vented homes and some commercial buildings become significantly contaminated by radon, a gas that carries a risk of causing human toxicity through the development of cancers, especially lung cancer.

Particulates are various sorts of solid or liquid materials that are small enough to be suspended in the atmosphere as fine dusts or aerosols. Particulate emissions inside of buildings are associated with smoke, physicalchemical deterioration of ducts, insulating materials, walls, ceiling tiles, and paints, fibers from clothing and other fabrics, and many other sources. Particulates may also be drawn into buildings along with unfiltered, ambient air. Particulates are aggravating to many people, who may develop irritations of the upper respiratory tract, such as asthma. Some chemicals contained in particulates, especially certain metals and polycyclic aromatic hydrocarbons, are widely regarded as toxic substances, and unnecessary exposures should generally be avoided. The particulate size range of 0.004-0.4 in (0.1-10 mm) is of particular importance in terms of human exposures, because this size range is efficiently retained in the deepest parts of the lungs. Particulates smaller than 0.004 in are generally re-exhaled, while particles larger than 0.4 in are trapped in the upper respiratory system and have little toxic effect.

Sometimes, microbial matter (or bioaerosols) can be an indoor-air problem. Usually, this involves spore-producing fungi that occur in damp places in the ventilation system, carpets, or other places. Many people have allergies to fungal spores, and can be made ill by excessive exposures to these bioaerosols in indoor air. Bioaerosols of other microbes such as yeast, bacteria, viruses, and protozoan may also be important problems in the atmosphere of buildings. On a rare occasion, pathogenic bacteria such as the Legionella associated with pneumonia-like Legionnaires' disease, can be spread through the ventilation system of buildings. Other potential pathogens in the inside air of buildings include the fungi Aspergillus fumigatus and Histoplasma capsulatum.