Cynthia Smith was poisoned silently and slowly in her own home every day as she worked, ate, read and watched TV.

Even as she slept, the colorless, tasteless, odorless poison attacked her brain, snatching parts of its functioning for two and a half years before the stealthy culprit was discovered.

Her faulty furnace leaked carbon monoxide — not enough to kill her, but more than enough to damage her brain, her health, her emotions, her job and her future.

Today, the former venture capitalist and 30-something businesswoman from San Francisco is dealing with brain damage, heart trouble and the possibility of other health problems after she unknowingly inhaled the poison for months on end.

Smith is in Salt Lake City to visit Dr. Lindell Weaver, medical director of the Hyperbaric Medicine Center at Intermountain Medical Center, whose most recent study on carbon monoxide poisoning was published Wednesday in the New England Journal of Medicine. He is one member of a team of doctors now helping Smith deal with the consequences of the poisoning, which she discovered in the fall of 2001.

Both Weaver and his patient wonder how many others are now being poisoned little by little, with children, unborn babies and older people at particular risk for damage with exposure to CO levels that may not do the same harm — or even create symptoms — in healthy adults.

The study describes the potential for ongoing impact on those suffering from long-term exposure, and advises that such patients "should be informed that they may not fully recover after poisoning." Patients not only have been deprived of oxygen that is critical to brain function, but the inflammation caused by CO results in injury to tissue that may explain symptoms in patients that develop long-term cognitive problems, the study said.

Yet it was the initial lack of specificity and severity of her symptoms that kept Smith from discovering the problem, and getting the necessary treatment more quickly, she said.

"I had nausea, headaches and general fatigue based on the time of exposure or whether I was out of town. If you were to create a list, there could be 20 to 30 symptoms," none of which specifically pointed to CO poisoning, she said. Loss of balance and a lack of concentration were also present.

She didn't associate her health problems with CO poisoning until she discovered the faulty furnace, and by that time, "there was so much impairment" in her ability to think and focus on her work, even to deal with e-mail, "that I didn't realize how much (impairment) there really was."

Of the 238 patients he has seen during the past several years, Weaver said from 10 to 15 percent were exposed to carbon monoxide for more than 24 hours, some of them for a few days and others for weeks or even months. Exposure can happen anywhere, but is most common at home or at work.

Higher-level brain function is often affected, depending on the level and length of exposure, and includes impairment in multi-tasking ability and short-term memory, he said. "Associated disorders include gait and motor disturbances, peripheral neuropathy, hearing loss and vestibular abnormalities, and dementia and psychosis, which can be permanent."

Carbon monoxide poisoning can also exacerbate angina and cause cardiac injury, the study said, recommending that patients with CO poisoning be followed by doctors over time, because the extent and rate of recovery vary and associated permanent health problems can develop weeks afterward.

Though the poisoning "can cause myriad neurological and neuro-psychological problems, the incidence of disorders after carbon monoxide poisoning is not clearly known," the study said. Nearly 20 percent of CO poisoning patients had cognitive problems and 37 percent had abnormal neurological evaluations six years after poisoning.

It's been more than seven years since Smith discovered the source of her symptoms. Yet early on, she had trouble finding doctors in San Francisco that recognized how the poisoning had impacted her. Many believed her problems were psychological, rather than physiological.

"As a professional with a background in Silicon Valley, I was accustomed to being heard. It was very humbling and very distressing" when her complaints were originally dismissed.

As she continues to experience cognitive problems created by the CO poisoning, she has changed jobs and modified her lifestyle to deal with the daily challenges. "My IQ hasn't changed, but the way my brain works around it has changed. … It may take me three times longer to complete a task and I have about one-third of the energy to do it."