Carbon Monoxide: The Current Situation April 1998 (AEN-196)

ISU Extension Publication #: AEN-196
Author: Dr. Thomas Greiner, Dept. of Agricultural and Biosystems Engineering
Iowa State University
Date: 4/98

Introduction

The number of persons known to be exposed to carbon monoxide has reached alarming proportions. Annually, carbon monoxide (CO) causes more than one-half of all poisoning deaths in the United States. Each year over 3,800 people die, and more than 10,000 individuals seek medical attention or miss work. Up to 40% of the survivors of acute CO poisoning develop memory impairment and other serious, permanent health problems. Carbon monoxide is a ubiquitous toxin that occurs during incomplete combustion. A California study found that in 5 to 10% of homes indoor CO concentrations exceeded the federal outdoor air standard of 9 parts per million (ppm). (There is no indoor residential air quality standard). In 30 to 40% of the California homes indoor CO levels were measurably higher than outdoor levels, indicating that many homes have indoor sources. The large number of residential carbon monoxide detectors alarming confirms that many persons are exposed to elevated levels of carbon monoxide in their homes. In 1996 MidAmerica Energy (a Des Moines utility) found elevated concentrations of carbon monoxide (above 20 ppm) in 1327 structures and during the 95-96 heating season Minnegasco (a Minnesota utility) responded to over 22,000 carbon monoxide calls (Wilber, 1997).

The public is concerned about carbon monoxide. Seventy-four percent (74%) of 2,198 responders to the 1997 Iowa Farm and Rural Life Poll rated carbon monoxide poisoning in homes a major to moderate risk. Only 7% thought CO poisoning was of little importance. In a survey of 106 Iowa fire departments, 94 agreed or strongly agreed with the statement that carbon monoxide poisoning is a serious problem.

There are a variety of reasons carbon monoxide problems occur: failure of heating systems, poor maintenance of heating systems and vents, tighter houses, depressurization of homes, failure of vent systems, and operating vehicles in garages. As houses become tighter to conserve energy, natural draft heating appliances that rely on buoyant forces are failing to vent. Often heating contractors, home builders, and other building professionals do not have the equipment or training to diagnose and correct the problems, placing the occupants at increased risk from carbon monoxide poisoning. Cases of medical misdiagnosis and heating technician failures abound.

Immediate and rapid action is needed to reduce the threat from carbon monoxide. Yearly service of heating appliances by qualified heating contractors, education efforts with professionals and the public, installation of U-L or IAS listed carbon monoxide detectors, and correction of CO problems by qualified individuals need to be the highest priorities. Long-term solutions include additional research, more education, licensing of professionals, and legislative action.

Carbon monoxide poisoning is preventable. Action is needed to reduce the needless cost, pain, suffering, and death CO causes.

Description

Carbon monoxide (CO) is a highly toxic colorless, odorless, tasteless, non-irritating gas produced when carbon-based fuels burn incompletely. CO is slightly lighter than air (0.97) and moves easily throughout an entire building via small cracks.

Sources

CO is produced by common home appliances, including: gas or oil furnaces, gas refrigerators, gas clothes dryers, gas ranges, gas water heaters, gas space heaters, fireplaces, charcoal grills, and wood burning stoves. The amount of CO in combustion products varies widely, from a few ppm from a properly operating gas burner to several percent from an improperly operating gas burner, charcoal and smoldering wood. Contrary to popular belief, carbon monoxide can, and often is, produced from a blue burning flame. Equipment must be used to determine proper combustion (Greiner, 1997a). Fumes from automobiles and gas-powered lawn mowers also contain carbon monoxide. Changes in design, proper maintenance, and catalytic converters can dramatically reduce CO from internal combustion engines. Cigarette smoke contains CO and smokers typically will have elevated concentrations of CO in their blood (Ellenhorn, 1988).

Dangers of Carbon Monoxide

Carbon monoxide is an extremely dangerous poison because it can not be seen, smelled, or tasted. It is rapidly absorbed by the lungs and quickly passes to the blood, depriving the body of oxygen. Early symptoms of CO poisoning mimic the flu, and may include headaches, fatigue, nausea, dizzy spells, confusion, and irritability. As CO levels increase vomiting and loss of consciousness may occur. Brain damage, heart, organ and tissue damage and death can result. Because CO reduces oxygen delivery to the brain (causing carbon monoxide intoxication), persons with elevated levels of CO in their blood do not think clearly, and fail to recognize the warning signs. Carbon monoxide also concentrates in myoglobin, causing ischemia and potentiating the hypoxia induced by impaired oxygen delivery (Ellenhorn, 1988).

Chronic and delayed health effects include visual loss, dementia, retardation, memory loss, personality changes, and concentration deficits. Delayed neurological sequelae occurs in 11% or more of the patients hospitalized for CO poisoning, resulting in mental deterioration, incontinence, gait disturbance, or mutism. CO poisoning not only results in short-term medical problems, but in many cases causes long-term conditions including permanent brain damage. (Ellenhorn, 1988). A young Iowa mother suffered permanent brain damage after exposure to carbon monoxide from a furnace. Six months after the exposure she was still unable to perform her work as a computer programmer, and lost her position (Greiner, 1995).

Extent of problem

Carbon monoxide is the leading cause of poisoning deaths in the U.S., with more than 3,800 people known to die annually from CO (accidental and intentional). Fire causes approximately two-thirds of known fatalities. Automobile exhaust and faulty heating equipment cause the remaining one-third. In addition to known deaths, more than 10,000 individuals seek medical attention or miss at least one day of work because of a sublethal exposure. Successful removal of CO from the blood does not ensure an uneventful recovery with no further clinical signs or symptoms. Neurological problems may develop insidiously weeks after an acute episode of CO poisoning, including: intellectual deterioration; memory impairment; and cerebral, cerebellar, and midbrain damage. Up to two-fifths (40%) of patients develop memory impairment and a third (33%) suffer late deterioration of personality. Arrhythmias are a common complication of CO poisoning. Conduction defects also are found, possibly from cardiomyopathies, but the precise mechanisms by which these occur are not understood. Other systemic complications, such as skeletal muscle necrosis, renal failure, blood dyscrasia, pulmonary edema, and hemorrhage in various tissues can also occur as a result of CO poisoning.” (EPA, 1992).

Carbon monoxide, even at low concentrations, results in damage over an extended period. Studies indicate the risk of dying from CO poisoning increases with age. The U.S. Consumer Product Safety Commission (USCPSC) reported there are more than 31 million people over the age of 64 in the U.S. and stated, “Since the elderly are increasing in the U.S. population, the issue of CO intoxication for this group should be of concern.” The USCPSC found several studies that observed small decreases in work capacity at carboxyhemoglobin (COHb) levels as low as 2.3 to 4.3%. They concluded “The cardiac compromised population is estimated to include more than 13.5 million people of which some unknown portion will experience toxicity at COHb levels as low as 2-3%.” (Burton, 1996) .

A 1995 study investigating the association between hospital admissions for congestive heart failure (CHF) in the elderly and air pollutants found ambient CO levels were positively associated with hospital admissions. The relative risk associated with an increase of 10 ppm in CO ranged from 1.10 in New York to 1.37 in Los Angeles. In the seven cities analyzed, approximately 3,250 hospital admission for congestive heart failure (5.7%) each year can be attributed to CO. The total annual cost of the admissions was $33 million (Morris, 1995).

A 1997 study found that CO monoxide interfered with the normal functions of nitric oxide, causing nitric oxide concentrations to increase quickly. (Thom, 1997). The author stated he believes “untold thousands” die from the accumulation of low-level carbon monoxide exposure.

The problem is without doubt larger than reported. The EPA notes “..not all instances of CO poisoning are reported and complete up-to-date data are difficult to obtain. Often the individuals suffering from CO poisoning are unaware of their exposure because symptoms are similar to those associated with the flu or with clinical depression. This may result in a significant number of misdiagnoses by medical professionals…Therefore, the precise number of individuals who have suffered from CO intoxication is not known, but it is certainly larger than the mortality figures indicate. Nonetheless, the reported literature available for review indicates the seriousness of this problem.”

Carbon monoxide poisoning can easily be misdiagnosed by medical personnel, as highlighted by a 1996 incident in Cleveland Ohio. Five persons went to the emergency room with flu-like symptoms. They asked if their symptoms could be caused by CO exposure. They were told they all had the flu and were sent home, where they died from CO poisoning. One study showed that 23.6% of persons presenting to a Kentucky hospital during February 1985 had elevated carboxyhemoglobin concentrations. None were initially diagnosed as suffering from carbon monoxide poisoning (Dolan, 1987).

Accidental carbon monoxide exposures can be sporadic and widely isolated. Small-scale short-term monitoring of ambient CO concentrations in homes is unlikely to discover these exposures. Still, a short-term California study found that in 5 to 10% of homes indoor carbon monoxide concentrations exceeded the federal air standards for outdoor air of 9 ppm. (There is no indoor residential air quality standard). In 30 to 40% of the California homes indoor levels were measurably higher than outdoor levels, indicating many homes have indoor sources. A total of 21 homes with high CO levels from indoor sources were selected for additional studies. Possible causes for elevated levels of CO included heating problems in 8 homes, kitchen range problems in 8 homes, vehicle exhaust from garages in 5 homes, and heavy smoking in 2 homes. (Note, some homes had multiple sources). The authors note “The reader is advised to consider the small number of residences sampled and the sampling frame… samplers were placed in the homes for only 48 hours. This data represents a two day snap shot…It is quite probable that the number of homes with exceedences would have increased if we had sampled for a longer period of time.” (Wilson, 1993 and 1995) (Colome, 1994).

Anecdotal accounts of carbon monoxide exposure abound. At one meeting 34 of the 64 heating contractors present (53%) responded that at some time they had been poisoned by carbon monoxide. At a training session for utility technicians 61 of 91 (65%) responded that they had been poisoned by CO. Several indicated the exposure had been from vehicle exhaust or defective heating appliances (Greiner, 1997. Unpublished program evaluation forms). A single carbon monoxide exposure during a lifetime can cause long-term illness or death. Just like any accident, the exposure is unexpected, unpredictable, and difficult to reproduce. The effects may change a person’s entire life without that person knowing what happened.

The public and professionals are concerned about carbon monoxide, as shown by attendance at CO meetings. For three years Iowa State University Extension and Des Moines Community College have organized annual carbon monoxide conferences, with an average attendance over 125. A single heating contractor meeting attracted over 140 persons, the largest attendance ever recorded. Persons attending Iowa State University CO meetings responded on evaluation forms that they feel CO is a significant problem (3.9 out of 4.0). In a 1997 survey of 106 Iowa fire departments, 94 agreed or strongly agreed with the statement that carbon monoxide poisoning is a serious problem. Seventy-four percent (74%) of 2,198 responders to the 1997 Iowa Farm and Rural Life Poll rated carbon monoxide poisoning in homes a major to moderate risk to the public. Only 7% thought CO poisoning was of little importance (Lasley, 1997).

Heating contractors, fire departments, health departments, and carbon monoxide investigators report the number of carbon monoxide incidents increase each year. A survey of Iowa heating contractors indicate they are responding to hundreds of carbon monoxide-related calls. Some contractors report they found carbon monoxide in all cases after a CO detector alarmed. Others report never finding CO after an alarm. The disparity raises serious questions (Greiner, 1996).

In 1995 MidAmerica Energy (Des Moines office) responded to 2023 request for carbon monoxide checks. They found elevated concentrations of CO (above 20 ppm) in 490 buildings. In 1996 the number of calls increased to 5017, with 1012 positive. In 1997 they had 5794 calls, with 1327 positive.

Minnegasco Utility, in Minneapolis/St.Paul responded to nearly 14,000 carbon monoxide calls during the 95-96 heating season. They found carbon monoxide in approximately 2,800 dwellings, representing a serious problem. They were concerned about homes where they did not find a source after a carbon monoxide detector had alarmed. They conducted a follow-up study of 50 homes where they originally had found no CO. In 49 of the homes the in-depth follow-up research study found sources of carbon monoxide missed during the initial utility technician response.

Iowa CO case investigations by Dr. Thomas Greiner revealed numerous instances of carbon monoxide exposure in Iowa dwellings. The reasons for elevated levels of CO in 29 cases proved to be numerous and varied. There were 3 deaths, 28 individuals who sought medical treatment, 41 individuals reported unconfirmed symptoms (headaches or flu-like symptoms), 1 dead pet, and 1 sick pet. Utility/heating contractors/ and fire departments did not typically find and correct the problem quickly, if ever. In only two instances did they correctly diagnose and correct the problem during their first visit. Dr. Greiner also found cases where medical personnel misdiagnosed CO problems. For example, one family was diagnosed with influenza by their physician. The following week the daughter suffered seizures. One week later the entire family was overcome by carbon monoxide from a malfunctioning furnace (Greiner, 1996a).

The number of utility company calls for CO increase in the winter months of November, December, January and February, usually peaking in January. Illnesses diagnosed as influenza cases also peak in winter months. Some influenza cases are, in fact, caused by carbon monoxide poisoning. As quoted in Ellenhorn’s text Medical Toxicology “The effects of carbon monoxide are protean and occasionally mimic other diseases. Subacute toxicity may be diagnosed as a result of its similarity to a flu-like syndrome (headache, dizziness, nausea, malaise, decreased exercise tolerance) or gastroenteritis, especially in children. Chest pain palpitations, visual disturbances, difficulty concentrating, and syncope were common complaints associated with chronic occult carbon monoxide exposure.”

During the winter of 1997-1998 the Linn County Health Department, Mercy Hospital, and St. Lukes Hospital (Cedar Rapids, Iowa), in cooperation with Iowa State University, performed non-invasive carboxyhemoglobin screening in the emergency room, with a follow-up home assessment for those who were identified as having high exposure. Preliminary results (with no error checking yet performed) indicate approximately 5 percent of those tested had elevated concentrations of carbon monoxide. The risks of carbon monoxide exposure are severe. Finding 5% with elevated CO justifies consideration of routine medical screening for CO. Missing a diagnosis of carbon monoxide poisoning can lead to permanent damage or death.

Carbon monoxide detector manufacturers estimate CO detectors are currently in only 5 to 10 percent of homes. As the number of CO detectors increase, the number of homes identified with elevated concentrations of carbon monoxide will increase. Previous medical data warns us that carbon monoxide is a serious toxin. Recent medical research reveals carbon monoxide inhalation is even more serious than was previously known.

The problems

Carbon monoxide problems can be caused by: engines run in attached garages; unvented gas appliances; improper heating system design; poor installation; improper modifications; deteriorated vent systems and furnaces; poor maintenance; inadequate combustion air, make-up air, and ventilation air; house depressurization; and blocked or failed chimneys.

Houses have been made “tighter” to conserve energy. Tight houses have fewer air leaks, with less air infiltration/exfiltration. Vented appliances and appliances that exhaust air must have adequate combustion and make-up air from outdoors. In most houses, air needed for proper combustion, for proper venting of the appliance, to replace exhausted air, and to dilute contaminants is assumed to be supplied by naturally occurring air leaks. These natural leaks are often not sufficient.

Anything that changes air flow can adversely affect the safe operation of natural draft heating appliances including: weather; house orientation, shape and size; exterior obstructions; vent location, height, size, and type; bathroom exhaust fans; vented kitchen range hoods; clothes dryers; size, shape, and location of air leaks; attic ventilation; vented gas appliances; wood-burning fireplaces; operation of furnace blower; and furnace return and supply ducts location and leakage. These factors interact in complex ways to cause vent failure. When the vent fails, combustion products enter living quarters.

Natural draft appliances depend on extremely small pressure differences to draft correctly. If the house is depressurized (operates under a negative pressure, or suction), the vent system can fail. Adding an exhaust fan or recessed ceiling lights, sealing basement windows, closing a supply register, or adding a return air can upset the balance, causing combustion products to spill into the home. In many instances no one takes responsibility for ensuring a house functions correctly, either during design, construction, maintenance, or modifications. There are few organizations or individuals with the training or equipment to either design or troubleshoot such systems.

Building codes, including the Uniform Mechanical Code, “assume” houses have a minimum of 1.0 air change per hour (ACH) from infiltration/exfiltration. They further “assume” that, unless the house is of “unusually tight construction”, adequate combustion air will enter through building leaks (UMC, 1994). Previously the assumption that houses are “loose” and have 1.0 air change per hour may have been valid for, but is not valid today. The California study found that single family detached homes with electric ranges had an average of only 0.36 ACH over the testing period. An average air change rate does not ensure adequate air change during all conditions. In the California houses, 90% had less than 1.0 ACH, 52% had less than 0.5 ACH, 30% had less than 0.35 ACH, 7% had less than 0.2 ACH, and two houses had 0.1 ACH. Other studies confirm that houses are tighter than 1.0 ACH (Persily, 1996).

Natural draft appliances rely on the buoyant force of hot gases and sufficient combustion air, a combination not always available in tight houses. Without sufficient combustion air, make-up air, and ventilation air, the following problems can occur: heating appliance vent systems spill or downdraft; multiple heating systems fight for combustion air (ie, operation of one appliance can cause downdrafting in another appliance); exhaust fans depressurize the structure, causing vent failure; fireplaces spill smoke and downdraft; insufficient dilution air allows contaminants levels to increase; and moisture levels increase, leading to mold and mildew growth. These problems are routinely found in Iowa houses and have led to numerous instances of carbon monoxide exposure (Greiner, 1996a).

Combustion products from improperly vented and unvented gas appliances (such as gas kitchen ranges and some gas fireplaces) are released directly into living spaces. Pollutants include: carbon monoxide, nitrogen dioxide, small inhalable particles, polycyclic aromatic hydrocarbons, and water. These can cause death; headache; fatigue; lung damage; lung disease; respiratory infections; nose, throat, and eye irritation; bronchitis; allergies; asthma, ear infections; and lung, stomach, bladder, and skin cancer (California Air Resources Board, 1994). Other studies show that operation of a vent-free (unvented) gas appliance causes pollutant levels to rise (DeWerth, 1996), sometimes to hazardous concentrations (Tsongas, 1994). In tighter houses the concentrations rise higher and remain longer.

Mechanical equipment can fail, and increase CO concentrations. Failure of equipment can not always be predicted or avoided by annual inspections. (Greiner, 1997).

Some of the changes in the home that can cause vent failure include: installing a kitchen exhaust fan, caulking and sealing windows, adding a wood-burning fireplace, adding an attic ventilation fan, adding a return air in the basement, changing furnace duct configuration, and remodeling the basement.Failures can result when building contractors fail to consider the home as a system. All individuals affecting the system must be vigilant to prevent carbon monoxide exposure. This includes occupants, home builders and remodelers; appliance and heating manufacturers; appliance and heating installers; and utility technicians.

In most houses with attached garages a portion of the air flow into the home comes from attached garages. Starting a vehicle in a garage (even with the outside garage door open) produces carbon monoxide that can remain in the garage and be drawn into the house over the course of several hours. In thirty-seven of fifty previously unexplained instances of CO, carbon monoxide from vehicles in garages was found to be one source (Minnegasco, 1997).

Conclusions

Exposure to carbon monoxide is high risk because:

  • CO is colorless, odorless, tasteless, and non-irritating.
  • CO poisoning reduces mental acuity.
  • CO symptoms mimic other common illnesses.
  • CO is often misdiagnosed by medical personnel.
  • CO is highly toxic. A single exposure in a lifetime can cause permanent damage or death.
  • CO at low concentrations can cause health problems, especially for individuals at risk.
  • CO exposure at low concentrations often occurs.
  • CO exposures can be accidental, sporadic, difficult to predict, and difficult to reproduce.
  • Responders to CO incidents do not always have adequate training or equipment.
  • The public recognizes the danger of death from carbon monoxide, but does not realize the health risks of lower doses.
  • The public does not recognize actions that can lead to carbon monoxide exposure (for example, do not recognize that warming up a vehicle in an open garage attached to the house can increase CO in the house).
  • There is misinformation about carbon monoxide symptoms and treatment (ie, medical staff who incorrectly use pulse oximeters to screen for CO or mistakenly believe patients exposed to CO will exhibit bright red skin).
  • There is misinformation about combustion (ie, many think a blue flame can not produce carbon monoxide).

The short-term solutions: The following actions reduce consumer risks of CO poisoning in their home:

  • Installation of U-L or IAS (International Approval Services) listed carbon monoxide detectors.
  • Yearly service of heating appliances by a qualified service technician.
  • Immediate action to protect all building occupants when a detector alarms.
  • Correction of problems after a carbon monoxide detector alarms.

Because many professionals lack equipment and training to adequately diagnose and correct carbon monoxide problems, home occupants with an alarming detector or a known carbon monoxide exposure often must be persistent in getting the problems diagnosed and corrected (Greiner, 1996a).

Long-term solutions: Some actions that reduce the risks of CO poisoning include:

  • Conducting research to understand the problem and/or develop methods of control.
  • Disseminating information and educating the public.
  • Initiating or increasing activities in regulations, permits, and enforcement of regulations and permits.
  • Taking legislative action at the federal, state, and local level.

Summary

Carbon monoxide poisoning causes severe and permanent health problems and death. It is preventable. Action is needed to reduce the needless cost, pain, suffering, and death CO causes.

Prepared by
Thomas H. Greiner, Ph.D., P.E.
Extension Agricultural Engineer

The Iowa Cooperative Extension Service’s programs and policies are consistent with pertinent federal and state laws and regulations on nondiscrimination regarding race, color, national origin, religion, sex, age and disability.

References

Burton, Laureen E., 1996. Toxicity from Low Level Human Exposure to Carbon Monoxide. U.S. Consumer Product Safety Commission Report, USCPSC, Bethesda, MD. July 1, 1996

California Air Resources Board, 1994. Combustion Pollutants in your Home, Cal. Envirn. Prot. Agency

Colome, S.D., A.L. Wilson, and Y. Tian, 1994. California Residential Indoor Air Quality Study. Volume 2 Carbon Monoxide and Air Exchange Rate: A Univariate & Multivariate Analysis. Irvine, CA: Integrated Environmental Services.

DeWerth, Douglas, 1996. Development of Sizing Guidelines for Vent-Free Supplemental Heating Products, AGA, Cleveland.

Dolan, Michael C. M.D, T.L Halton, G.H Barrows, C.S. Short, and K.M. Ferrell, 1987. Carboxyhemoglobin Levels in Patients with Flu-Like Symptoms. Annals of Emergency Medicine 16:7.

Ellenhorn, Mathew J. and Donald G. Barceloux, 1988. Medical Toxicology: Diagnosis and Treatment of Human Poisoning. Elservier Publishing, New York.

Environmental Protection Agency (EPA), 1992. Air Quality Criteria for Carbon Monoxide, EPA/600/8-90/045F, June 1992).

Greiner, Thomas H., 1995. Phone conversation with victim of carbon monoxide poisoning. From unpublished case study. Iowa State University Extension Service, Ames, Iowa.

Greiner, Thomas H., 1996. Survey of Heating Contractors. Unpublished report, Iowa State University Extension Service, Ames, Iowa.

Greiner, Thomas H., 1996a. Carbon Monoxide Detectors, Responding to II. analysis of Unexplained CO Detector Alarm Activation. Prepared for the U.S. Consumer Product Safety Commission Hearings February 21 and 22, 1996, U.S. Consumer Product Safety Commission, Bethesda, Maryland.

Greiner, Thomas H., 1997. Carbon Monoxide Problems from New Furnaces, Home Energy, May/June, 1997

Greiner, Thomas H. 1997a. Carbon Monoxide Poisoning: Checking for Complete Combustion. AEN-175. Iowa State University Agricultural Engineering Note, Ames, Iowa.

Lasely, Paul, 1997. Iowa Farm and Rual Life Poll, Pm-1745. Iowa State University Extension, Ames, Iowa

Minnegasco, 1997. Report on Undiagnosed Carbon Monoxide Complaints, Advanced Certified Thermography, Lakeland Shores, Minnesota

Morris, Robert D., Elena N. Naumova, and Rajika L. Munasinghe, 1995. AJPH; 85:10, pg. 1361-1365.

Persily, Andrew K., 1996. Carbon Monoxide Dispersion in Residential Buildings: Literature Review and Technical Analysis. NISTIR 5906. National Institute of Standards and Technology, Gaithersburg, MD

Thom, Stephen R., Y.Anne Xu, and Harry Ischiropoulos, 1997. Vascular Endothelial Cells Generate Peroxynitrite in Response to Carbon Monoxide Exposure. Chem. Res. Toxicol; 10, 1023-1031.

Tsongas, George, 1994. Field Monitoring of Elevated CO Production from Residential Gas Ovens, ASHRAE IAQ “94, St. Louis, Missouri

UMC (Uniform Mechanical Code), 1994. Dwelling Construction Under the UMC

Wilber, Matt, 1997. Carbon Monoxide Round Table Discussion. U.S. Consumer Product Safety Commission, January 23, 1997. USCPSC, Bethesda, Maryland.

Wilson, A.L., S.D. Colome, and Y. Tian, 1993. California Residential Indoor Air Quality Study. Volume 1 Methodology and Descriptive Statistics. 1995 Volume 3 Ancillary and Exploratory Analysis.