In June 2001, a middle-aged couple with a history of smoking moved into a newly built, two-story, wood-framed home with a finished basement and adjacent crawlspace on land that was formerly mined. Shortly after occupancy, the 42-year old woman noted new-onset shortness of breath, lightheadedness, dizziness, and fatigue while in the basement. Within a few months, the previously healthy 42-year old man reported new-onset mild confusion, poor concentration, headache, and blurry vision while fabricating fishing poles in his basement workshop. These symptoms are consistent with exposure to elevated carbon dioxide and low oxygen concentrations. Their symptoms always resolved within minutes after leaving the basement. NIOSH received a technical assistance request in December 2003, from the West Virginia Department of Environmental Protection (WVDEP) to assist with the investigation of a residence where they found an oxygen-deficient environment in the basement and crawlspace areas. Carbon monoxide, methane, and 33 other gases were not detected (using an AIM Model 3250, IST-AIM, Horseheads, NY, USA), which led investigators to suspect that carbon dioxide could be displacing oxygen in the basement and crawlspace. NIOSH provided sampling equipment and safety and health information to WVDEP. WVDEP investigators found elevated carbon dioxide concentrations in the home. In addition, NIOSH conducted medical interviews with the homeowners and advised remediation workers about hazards and requirements for confined space entry. NIOSH informed the local and state health departments about the elevated carbon dioxide concentrations at this residence. A modified direct-reading carbon dioxide monitor (detection range up to 50% carbon dioxide) was used for short-term sampling from December 2003 until February 2004. Concentrations of carbon dioxide in the crawlspace were as high as 9.5% in the crawlspace, 11% in the crawlspace gravel, and 12% in the floor drain (outside air has a carbon dioxide concentration of 0.035%). Carbon dioxide levels on the upper floors exceeded the upper limit of detection (1%) of the standard carbon dioxide monitor. Oxygen concentration in the basement was intermittently deficient during the evaluation, with levels measuring as low as 14% in the crawlspace. In addition, WVDEP investigators took a soil gas sample and an air sample from a mine drainage pipe for carbon isotopic composition analysis, which led to the determination that the source of the carbon dioxide was likely from a carbonate source. Limestone, a carbonate, is the most likely source on this land since it is generally used during the remediation of surface mines. NIOSH recommendations to homeowners with symptoms consistent with carbon dioxide exposure or low oxygen concentrations include: 1. Determine whether the home is located above an underground coal mine or a reclaimed surface coal mine and then determine whether there could be potential carbon dioxide gas infiltration by using carbon dioxide meters. 2. Implement mitigation techniques to reduce infiltration, if found. 3. Monitor symptoms and take action, and 4. Consider installation of carbon dioxide monitors in the affected spaces. NIOSH recommendations to state and local agencies include: 1. Alert clinicians, especially those in emergency departments, to the symptoms associated with elevated carbon dioxide exposure, 2. Alert residents living on underground and/or reclaimed surface mines to the potential hazards of mine gas infiltration into their homes, 3. Alert utility workers and first responders to the existence of potentially hazardous air environments (e.g., oxygen deficiency, elevated carbon dioxide) in enclosed spaces, and 4. Consider instituting a program to assist residents in assessing risks and hazards of mine gas infiltration into their homes and providing information on the mitigation of these gases. The couple had symptoms consistent with exposures to elevated carbon dioxide levels. Their chronic exposures to carbon dioxide were likely greater than the NIOSH 8-hour Recommended Exposure Limit of 5000 ppm (0.5%) carbon dioxide and the 15-minute short-term exposure limit of 30,000 ppm (3%) while in the basement. The dramatically elevated carbon dioxide levels in this home are ground-related, as shown by the high levels in the crawlspace gravel, basement floor drain, and in soil samples surrounding the home. Sources of carbon dioxide from abandoned mines include decaying timber in mine spoil, acid mine drainage reactions with limestone, and emissions from the coal itself. CO2 concentrations in excess of 25% and oxygen concentrations as low as 10% have been recorded in homes above coal mines in Pennsylvania and in Great Britain. Preventive construction for mining-related indoor air quality problems includes sealing cracks, maintaining positive pressure in relation to the ground, and ventilating subsurface areas in a manner similar to that used for radon mitigation. Gas companies, first responders, and health providers should be aware of the possibility of oxygen deficiency and carbon dioxide toxicity when occupants of buildings in areas with reclaimed or abandoned coal mines complain of shortness of breath, palpitations, dizziness, confusion. Another sign of oxygen deficiency or elevated carbon dioxide concentrations is the difficulty maintaining a pilot light on a gas appliance such as a water heater. In affected homes, homeowners, indoor workers, public utility workers, emergency response workers, and remediation workers may be at risk from dangers associated with confined space entry. This dictates increased public awareness and special training for workers, careful measurement of the environment to assess potential risks, precautions to avoid incapacitation, and preparation for rescue should a worker encounter immediately dangerous conditions.