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Soot, char and ash: more toxic than you think

When you think of charred wood, ash or soot, you might think that they are little more than harmless by-products of incomplete combustion. Afterall, haven’t people cooked their food and kept warm by burning wood in their stoves and fireplaces for millennia? 

When you see photos taken during the overhaul phase of structure fires it is not uncommon to see firefighters working without their BAs, breathing in toxic particulate, and their faces covered in soot. Images like these gives the impression that post-fire environments are of little concern and relatively safe. However, this is far from the truth.

Risks of exposure to burned wood

The World Health Organization estimates that roughly 4 million people worldwide die prematurely each year from diseases caused by the domestic burning of wood.1 The WHO states: ‘While the situation in developing countries, where people often cook over open fires in their homes, is more dire, wood burning is nonetheless becoming a major public health concern in the developed world as well. According to a 2016 report by the Danish Ecological Council, pollution from wood burning now contributes to approximately 50% of all health damages. Pollution from wood burning is now the most health damaging and expensive environmental problem in Denmark, causing several times more harm than fine particles from domestic road traffic.’2,3 

That being said, maybe there’s more to the story about soot, char and ash that firefighters need to be aware of.

Soot and Sir Percivall Pott’s discovery 

Sir Percivall Pott (1714–1788) was an English surgeon and the first scientist to demonstrate that cancer may be caused by an environmental carcinogen. In 1775, Pott found a correlation between chimney sweeps’ exposure to soot and a high incidence of a condition called ‘soot wart’, a cancer later found to be squamous cell carcinoma. The cancer primarily affected chimney sweeps who had been in contact with soot since their early childhood. 

In the 1700s and early 1800s, boys (often orphans) as young as 4 years of age, known as ‘climbing boys’, were used as chimney sweeps due to their small size, which allowed them to fit inside chimney flues. During this time in history, most people burned wood in their fireplaces to keep warm. It wasn’t until the early 1800s that coal was being widely used. 

Boys as young as 8 years old contracted the disease and many suffered an array of other debilitating illnesses resulting in respiratory and cardiovascular ailments and even blindness. It is believed that dermal absorption, inhalation and ingestion of soot and creosote triggered the diseases. 

Pressure treated lumber 

Buildings throughout the world that are constructed with wood often contain pressure-treated lumber. Because of its ability to resist rot, fungi and termites, it is commonly used where wood comes into contact with concrete floors or masonry walls or when it is exposed to the elements such as outdoor fencing, decks and picnic tables. In areas where termites are a major problem, treated lumber is used extensively, such as Hawaii where termites cause roughly $100 million dollars in damage annually.4,5 

Prior to 2004, pressure-treated lumber used in the U.S., Europe and other countries was manufactured with a chemical known as Chromated Copper Arsenate (CCA), which contained arsenic. Arsenic is a toxic heavy metal classified by the EPA as a known carcinogen. Exposure to arsenic can cause a wide range of diseases, including cancers of the lungs, skin, liver, kidney, prostate and bladder. CCA contains 47.5% hexavalent chromium, 18.5% copper and 34% inorganic arsenic. Even if pressure-treated wood doesn’t contain CCA, it still contains other hazardous insecticide and fungicide chemicals. 

Ammoniacal Copper Zinc Arsenate (ACZA) is a newer formula and successor of CCA. It consists of copper, zinc and arsenic. Unfortunately, ACZA is also harmful when burned, and exposure to it increases the risk of chronic respiratory disease and cancer.6,7 Here is a Safety Data Sheet that supports the fact that ACZA is carcinogenic:

When pressure-treated wood burns, the heat does not destroy the arsenic it contains. When this wood burns, it releases the chemical bond that holds the arsenic in the wood where it becomes part of the soot and ash. According to the Journal of American Medical Association, even minute amounts of the ‘fly ash’ from burning treated wood can have serious health consequences. 

In 1982, The Journal of the American Medical Association reported on a family in Wisconsin that burned CCA treated wood. All the family members suffered severe recurring nosebleeds, extreme fatigue and debilitating headaches. The parents complained about ‘blacking out’ for periods of several hours, followed by extended periods of extreme disorientation. The two youngest children had multiple seizures described as grand mal from birth to 1 year. They displayed recurrent ‘measles-like’ rashes on their bodies, as well as reddened thickened skin on the soles of their feet. Most striking was recurring seasonal alopecia, ranging from thinning of the hair in the parents to complete baldness in the youngest children. The symptoms were traced to breathing minute amounts of arsenic-laden dust. 

Firefighters are exposed to innumerable toxic substances during fire suppression. Image courtesy of Budgme (
Firefighters are exposed to innumerable toxic substances during fire suppression. Image courtesy of Budgme (

It was later discovered that the home had been heated with a small wood stove in which pressure-treated wood was burned. The ashes from the stove spread throughout the home, and test results showed that the ash contained more than 1,000ppm of arsenic. (Source: The Journal of the American Medical Association May 11, 1984, Volume 251 (JAMA 1984;251:2393–2396))8,9

Firefighters should be aware that although CCA-treated wood for residential use was banned in many countries in 2004, risk of exposure to arsenic-laden particulate and ash remains high in commercial buildings or older homes where treated lumber has burned, especially in the form of fine or ultrafine airborne particulate. When inhaled, ultrafine particles behave like gases and pass through the lungs directly into the bloodstream. These particles can also travel through the nose and affect the central nervous system via the olfactory nerve. 

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