Carbon monoxide detectors were the second item we bought for our home, right after new door locks.

I’d seen all the warnings on the news: Carbon monoxide—an odorless, colorless gas that can travel from faulty gas stoves, furnaces, or anything that burns fuel—was a silent killer that could slip into your home unnoticed.

To help sound the alarm if such a threat was present, we installed CO detectors on each level of our two-story house—one in our first-floor hallway, the other near our bedrooms upstairs. I also changed the batteries twice a year as directed.

Despite these precautions, I got carbon monoxide poisoning. Here's why:

How carbon monoxide entered our home

In autumn about six years after buying our home, I noticed black soot coating the walls around the heating vents throughout our house. I meant to call the heating repairman about it, but it slipped my mind.

In fact, a lot fell through the cracks that fall. My 4-year-old daughter had just begun a new nursery school, and I’d just quit a full-time job to pursue a career as a freelancer. After setting up my home office, and eager to get money coming in, I said yes to every project, no matter the size, clients threw my way. I was burning the candle at both ends, so it was no big surprise that I felt exhausted.

But it was more than that: One afternoon my husband found me standing in front of the refrigerator, staring inside like I’d lost something. When he questioned me, I turned and admitted that I had no idea what I was looking for.

“Did you want something to eat?” he asked.

I shook my head. I'd also recently lost my appetite.

Unbeknown to us at the time, I was showing classic signs of CO poisoning: confusion, nausea, and exhaustion. The fatigue made the smallest tasks seem insurmountable. I’d wake each morning with a headache. After dropping my daughter off at nursery school, I’d return home, settle into my desk, and zone out in front of my computer, sometimes falling asleep with my cheeks pressed into the keyboard.

I stopped meeting job deadlines, and for the first time in my life, I lost a job because of stupid mistakes. All in all, I thought I was losing my mind and intended to call the doctor—but I kept forgetting to make the call.

'You need to open the windows now'

About a month after I first noticed the black soot near our vents, my husband saw the same smudges and called our heating repairman. He was worried our furnace wasn't working properly and could be a fire hazard. The repairman hadn’t been in our basement for more than five minutes when he marched back upstairs.

“You need to open the windows now,” he said. “Your furnace is broken and leaking carbon monoxide. It’s in your air. It’s dangerous!”

I ran around the house and shoved all the windows open. Afterward, I told the repairman our home had two CO detectors, whose batteries I changed regularly. I asked how could we possibly have carbon monoxide in the air?

CO detectors, he replied, are good for only five to seven years. Ours were now 6 years old, and it turns out I should have replaced them a year earlier.

Then I became worried about the health of my family. Our doctors advised us to get our blood tested for the presence of CO. The tests were negative for my husband and daughter, both of whom are away from home during the day. I, however, wasn't so lucky. I had elevated levels of CO in my blood—which explained the symptoms I had been experiencing for weeks. My doctor said my working from home—and thus breathing in the poisonous air 24/7—was likely to blame.

The Centers for Disease Control estimates that each year about 400 Americans die from CO poisoning, and 4,000 more are hospitalized. Fortunately the levels in my body didn’t require hospitalization, but it did take several weeks of breathing clean air before I felt like myself again.

Replacing our faulty furnace and leaving the windows open for several days helped the CO dissipate in our home. And now, we have new CO detectors in the house, each tagged with a piece of masking tape with the date of purchase noted. We plan to replace them after four and a half years, which is sooner than the recommended six years, but I'd rather be safe than sorry.