Carbon Monoxide Protection with Woodstoves

By October 12, 2011 September 9th, 2020 Safety

If you’re like me, falling leaves with shorter days and longer nights only preambles coming months of cold and snow. If you’re like most, you’re concerned about increased energy costs and how you’ll efficiently heat your home this winter. How to conserve energy while keeping warm during the colder months is a major concern for most energy conscious people. And most everyone has their view on which way is best. For wife and I, we have spent much money, time and hard work to weatherize our home with insulating and sealing materials and doors. We are gradually eliminating the energivores in our home with higher efficiency appliances and equipment – like our woodstove. Our most recent investment should benefit us with 70.1% efficiency with a heat output of 68000 BTUs for an area of 800 to 2000 ft2. Since it is Environmental Protection Administration (EPA) approved and certified, I should expect it to produce from 2 to 7 grams of smoke per hour. Compared to our old non EPA certified stove that released, I’m assuming, 15 to 30 grams of smoke per hour, we’ve taken a step in the right direction.

Wood burning stove

I’m happy with our decision to keep the electricity bill low and burn wood this winter in our new stove. How we’ll stay warm during those cold nights is one less concern. Though I know that smoke from burning organic matter (i.e. wood) is made up of a complex mixture of dangerous gases and particles. I also know that during our renovations a polyethylene plastic vapor barrier was installed on the warm side of the insulation in walls and ceiling. The house in effect, is now sealed in a plastic bag controlling air intake and leakage. With higher health concerns, despite expected lower emissions with our new stove, we started to question the advantages of wood as a heat source under conditions of enhanced building efficiency. Health wise, were we better off with our old stove in a drafty room?

A major health threat from wood smoke comes from the fine microscopic particles that can get into your eyes and respiratory system. The size of particles is directly linked to their potential for causing health problems. Small particles <10 micrometers in diameter pose the greatest problems, because they can get deep into the lungs, and some may even get into the bloodstream. Finer particles ≤2.5 micrometers in diameter can affect both your lungs and heart.

Residential wood combustion emissions also contain sulfur oxides, nitrogen oxides, carbon monoxide and potentially carcinogenic compounds including polycyclic aromatic hydrocarbons, benzene, formaldehyde and dioxins. Of particular concern to me is carbon monoxide (CO) – an odorless, colorless gas that can cause sudden illness and death referred to infamously as “the silent killer”. Our red blood cells pick up CO quicker than they pick up oxygen. In fact, CO combines reversibly with the oxygen-carrying sites on the hemoglobin molecule with an affinity ranging from 210 to 240 times greater than that of oxygen. If there is a lot of CO in the air, the carboxyhemoglobin thus formed is unavailable to carry oxygen. This blocks oxygen from getting into the body (a condition known as tissue hypoxia) which can damage organ tissues and result in death. At lower concentrations, CO poisoning can be difficult to diagnose because the symptoms mimic other illnesses like gastroenteritis (nausea and vomiting). Other most common symptoms of CO poisoning are headache, dizziness, weakness, chest pain, confusion, loss of consciousness and death. High levels of CO inhalation can cause Cerebral edema (swelling of the brain) symptoms of which are delayed neurological problems that involve the “higher” or cognitive functions, and may cause a Parkinson-like brain syndrome. People who are sleeping or intoxicated can die from CO poisoning before ever experiencing symptoms.

To protect us from CO poisoning, I purchased a CO alarm that features a digital display which shows levels of CO in PPM (parts per million). The model I selected operates on an electrochemical sensor which has advantages over other technologies in that it has a highly sensitive and accurate linear output to CO concentration. Since it requires electrical power in continuous supply, it plugs into a 120V, 15 amp wall receptacle not controlled by a switch or dimmer. I made sure the CO alarm had a self-recharging battery backup that offers continuous protection in case of a power failure. The location of the electrical outlet is important. It must be heard from sleeping and living areas, making the hallway outside our bedroom the ideal place. The vapor density of CO with respect to air (Air = 1), at 1 atmosphere and 21°C (70°F), is 0.968. Since the vapor density of CO is roughly that of air, installing the alarm on the wall at eye level allows for optimum monitoring of the digital display.

Satisfied with the quality of my CO alarm’s technology and location of its installation, performance specifications became my next issue. I made sure it was tested and certified under an accredited independent product safety certification organization. The standards organizations of the United States, Underwriters Laboratories (UL), and Canada, Canadian Standards Association (CSA), have coordinated the writing of CO standards and product testing under standards UL2034 – Single Station Carbon Monoxide Alarm and CAN/CSA-6.19-01 (R2006) – Residential Carbon Monoxide Alarming Devices. The following table summarizes these safety standards for CO alarms.

CO concentration (ppm)


0 – 2

Normal conditions in and outside houses


Recommended exposure limit over a 24-hour period


Recommended exposure limit over a 1-hour period


CO detectors are not allowed to sound alarm unless this concentration is maintained for more than 30 days


CO detectors must sound alarm within 1 to 4 hours


CO detectors must sound alarm within 10 to 50 minutes


Slight headache, fatigue, dizziness and nausea after 2 to 3 hours.  CO detector alarm must sound within 35 minutes


CO detectors must sound alarm within 4 to 15 minutes


Dizziness, nausea and convulsions within 45 minutes, death within 2 to 3 hours


Death within 1 hour


Danger of death after 1 to 3 minutes

Knowing that safety standards are developed through consensus by committees of affected stakeholders that may include representatives from industry, governments, academia and the public interest, mine became a mission of inquisition how well this CO alarm device would protect my home’s occupants. I started by researching the several government and professional organizations that have posted recommended exposure limits to CO (mostly for the work place). The following table summarizes what I found.

Limit / Level Type Organization Industrial / Area
9 ppm TWA (1 Hour) US Environmental Protection Agency (EPA) General
9 ppm TWA (8 Hours) World Health Organization (WHO) General (Outdoor)
9 ppm Ceiling American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) General (Living Areas)
25 ppm TLV (8 Hours TWA) American Conference of Governmental Industrial Hygienists (ACGIH) General
35 ppm TWA (8 Hours) National Institute for Occupational Safety and Health (NIOSH) General
35 ppm TWA (8 Hours) US Environmental Protection Agency (EPA) General
50 ppm OSHA PEL
(8 Hours TWA)
Occupational Health and Safety Administration (OSHA) General
50 ppm OSHA PEL
(8 Hours TWA)
Occupational Health and Safety Administration (OSHA) Construction
50 ppm OSHA PEL
(8 Hours TWA)
Occupational Health and Safety Administration (OSHA) Maritime
125 ppm Excursion Limit (EL) American Conference of Governmental Industrial Hygienists (ACGIH) General
200 ppm Ceiling National Institute for Occupational Safety and Health (NIOSH) General

In reviewing this latter table, keep in mind that a Time-Weighted Average (TWA), unless otherwise specified, is for a normal 8-hour workday or 40-hour workweek. In an average week, we spend more than 23% of our time at home. Remember too that a Ceiling limit is one that may not be exceeded for any period of time, and an Excursion Limit (EL) is an ACGIH term that refers to the Ceiling limit for a short period of time (typically 15 – 30 minutes).

Comparing these two tables, my conclusions are not comforting – the concentrations per the standards (supposedly intended to protect us) are significantly in excess of the health guidelines. Further questioning revealed that, as of 2010, these most recent standards require the alarm to sound at higher levels of CO than with previous editions of the standard! The reason behind these changes (the prohibition of showing CO levels <30ppm on digital displays, and new alarms will not sound at CO concentrations up to 70ppm) is to reduce calls to fire stations, utilities and emergency response teams when the levels of CO are not life threatening. This change was also intended to reduce the number of calls to these agencies due to detector inaccuracy or the presence of other gases.

Low-level CO detection products are becoming commercially available. They will not be certified to CSA or UL standards, as these standards currently (as I’ve shown above) prohibit low-level displays. CO alarms with a digital display and a “history” option can provide the true CO concentrations in a house. A low-level display would be useful for people with existing respiratory problems or for those who like to spot evolving problems, rather than having to wait for the situation to become serious.

Now that my home is weatherized, more energy efficient, it is also more sensitive to depressurization when air is exhausted from the house. Because it’s tightly sealed, there are few holes to allow replacement air to enter, and the house pressure becomes negative compared to atmospheric pressure outside. This negative pressure works against chimney draft. Pollutants from fireplaces and woodstoves with no dedicated outdoor air supply can be “back-drafted” from the chimney into the living space, particularly in weatherized homes. As it turns out, the simplest solution is often the best – a directly-ducted source of outdoor air, only enough to compensate for air removed by continuously operating woodstoves for combustion.