Wildfire

Natural causes

Natural occurrences that can ignite wildfires without the involvement of humans include lightning, volcanic eruptions, sparks from rock falls, and spontaneous combustions.^[22][23]

Human activity

Sources of human-caused fire may include arson, accidental ignition, or the uncontrolled use of fire in land-clearing and agriculture such as the slash-and-burn farming in Southeast Asia.^[24] In the tropics, farmers often practice the slash-and-burn method of clearing fields during the dry season.

In middle latitudes, the most common human causes of wildfires are equipment generating sparks (chainsaws, grinders, mowers, etc.), overhead power lines, and arson.^{[25][26][27][28][29]}

Arson may account for over 20% of human caused fires.^[30] However, in the 2019–20 Australian bushfire season "an independent study found online bots and trolls exaggerating the role of arson in the fires."^[31] In the 2023 Canadian wildfires false claims of arson gained traction on social media; however, arson is generally not a main cause of wildfires in Canada.^[32][33] In California, generally 6–10% of wildfires annually are arson.^[34]

Coal seam fires burn in the thousands around the world, such as those in Burning Mountain, New South Wales; Centralia, Pennsylvania; and several coal-sustained fires in China. They can also flare up unexpectedly and ignite nearby flammable material.^[35]

Intensity variations during day and night

Intensity also increases during daytime hours. Burn rates of smoldering logs are up to five times greater during the day due to lower humidity, increased temperatures, and increased wind speeds.^[68] Sunlight warms the ground during the day which creates air currents that travel uphill. At night the land cools, creating air currents that travel downhill. Wildfires are fanned by these winds and often follow the air currents over hills and through valleys.^[69] Fires in Europe occur frequently during the hours of 12:00 p.m. and 2:00 p.m.^[70] Wildfire suppression operations in the United States revolve around a 24-hour fire day that begins at 10:00 a.m. due to the predictable increase in intensity resulting from the daytime warmth.^[71]

Increasing risks due to climate change

Climate change promotes the type of weather that makes wildfires more likely. In some areas, an increase of wildfires has been attributed directly to climate change.^[11]: 247 Evidence from Earth's past also shows more fire in warmer periods.^[74] Climate change increases evapotranspiration. This can cause vegetation and soils to dry out. When a fire starts in an area with very dry vegetation, it can spread rapidly. Higher temperatures can also lengthen the fire season. This is the time of year in which severe wildfires are most likely, particularly in regions where snow is disappearing.^[75]

Weather conditions are raising the risks of wildfires. But the total area burnt by wildfires has decreased. This is mostly because savanna has been converted to cropland, so there are fewer trees to burn.^[75]

Climate variability including heat waves, droughts, and El Niño, and regional weather patterns, such as high-pressure ridges, can increase the risk and alter the behavior of wildfires dramatically.^[76][77][78] Years of high precipitation can produce rapid vegetation growth, which when followed by warmer periods can encourage more widespread fires and longer fire seasons.^[79] High temperatures dry out the fuel loads and make them more flammable, increasing tree mortality and posing significant risks to global forest health.^[80][81][82] Since the mid-1980s, in the Western US, earlier snowmelt and associated warming has also been associated with an increase in length and severity of the wildfire season, or the most fire-prone time of the year.^[83] A 2019 study indicates that the increase in fire risk in California may be partially attributable to human-induced climate change.^[84]

In the summer of 1974–1975 (southern hemisphere), Australia suffered its worst recorded wildfire, when 15% of Australia's land mass suffered "extensive fire damage".^[85] Fires that summer burned up an estimated 117 million hectares (290 million acres; 1,170,000 square kilometres; 450,000 square miles).^[86][87] In Australia, the annual number of hot days (above 35 °C) and very hot days (above 40 °C) has increased significantly in many areas of the country since 1950. The country has always had bushfires but in 2019, the extent and ferocity of these fires increased dramatically.^[88] For the first time catastrophic bushfire conditions were declared for Greater Sydney. New South Wales and Queensland declared a state of emergency but fires were also burning in South Australia and Western Australia.^[89]

In 2019, extreme heat and dryness caused massive wildfires in Siberia, Alaska, Canary Islands, Australia, and in the Amazon rainforest. The fires in the latter were caused mainly by illegal logging. The smoke from the fires expanded on huge territory including major cities, dramatically reducing air quality.^[90]

As of August 2020, the wildfires in that year were 13% worse than in 2019 due primarily to climate change, deforestation and agricultural burning. The Amazon rainforest's existence is threatened by fires.^{[91][92][93][94]} Record-breaking wildfires in 2021 occurred in Turkey, Greece and Russia, thought to be linked to climate change.^[95]

Carbon dioxide and other emissions from fires

The carbon released from wildfires can add to greenhouse gas concentrations. Climate models do not yet fully reflect this feedback.^[17]: 20

Wildfires release large amounts of carbon dioxide, black and brown carbon particles, and ozone precursors such as volatile organic compounds and nitrogen oxides (NOx) into the atmosphere.^[96][97] These emissions affect radiation, clouds, and climate on regional and even global scales.^[16] Wildfires also emit substantial amounts of semi-volatile organic species that can partition from the gas phase to form secondary organic aerosol (SOA) over hours to days after emission. In addition, the formation of the other pollutants as the air is transported can lead to harmful exposures for populations in regions far away from the wildfires.^[98][16] While direct emissions of harmful pollutants can affect first responders and residents, wildfire smoke can also be transported over long distances and impact air quality across local, regional, and global scales.^[99]

The health effects of wildfire smoke, such as worsening cardiovascular and respiratory conditions, extend beyond immediate exposure, contributing to nearly 16,000 annual deaths, a number expected to rise to 30,000 by 2050. The economic impact is also significant, with projected costs reaching $240 billion annually by 2050, surpassing other climate-related damages.^[100]

Over the past century, wildfires have accounted for 20–25% of global carbon emissions, the remainder from human activities.^[101] Global carbon emissions from wildfires through August 2020 equaled the average annual emissions of the European Union.^[102] In 2020, the carbon released by California's wildfires was significantly larger than the state's other carbon emissions.^[103]

Forest fires in Indonesia in 1997 were estimated to have released between 0.81 and 2.57 gigatonnes (0.89 and 2.83 billion short tons) of CO₂ into the atmosphere, which is between 13–40% of the annual global carbon dioxide emissions from burning fossil fuels.^[104][105]

In June and July 2019, fires in the Arctic emitted more than 140 megatons of carbon dioxide, according to an analysis by CAMS. To put that into perspective this amounts to the same amount of carbon emitted by 36 million cars in a year. The recent wildfires and their massive CO₂ emissions mean that it will be important to take them into consideration when implementing measures for reaching greenhouse gas reduction targets accorded with the Paris climate agreement.^[106] Due to the complex oxidative chemistry occurring during the transport of wildfire smoke in the atmosphere,^[107] the toxicity of emissions was indicated to increase over time.^[108][109]

Atmospheric models suggest that these concentrations of sooty particles could increase absorption of incoming solar radiation during winter months by as much as 15%.^[110] The Amazon is estimated to hold around 90 billion tons of carbon. As of 2019, the earth's atmosphere has 415 parts per million of carbon, and the destruction of the Amazon would add about 38 parts per million.^[111]

Some research has shown wildfire smoke can have a cooling effect.^{[112][113][114]}

Research in 2007 stated that black carbon in snow changed temperature three times more than atmospheric carbon dioxide. As much as 94 percent of Arctic warming may be caused by dark carbon on snow that initiates melting. The dark carbon comes from fossil fuels burning, wood and other biofuels, and forest fires. Melting can occur even at low concentrations of dark carbon (below five parts per billion)".^[115]

Local sensor networks

A small, high risk area that features thick vegetation, a strong human presence, or is close to a critical urban area can be monitored using a local sensor network. Detection systems may include wireless sensor networks that act as automated weather systems: detecting temperature, humidity, and smoke.^{[140][141][142][143]} These may be battery-powered, solar-powered, or tree-rechargeable: able to recharge their battery systems using the small electrical currents in plant material.^[144] Larger, medium-risk areas can be monitored by scanning towers that incorporate fixed cameras and sensors to detect smoke or additional factors such as the infrared signature of carbon dioxide produced by fires. Additional capabilities such as night vision, brightness detection, and color change detection may also be incorporated into sensor arrays.^{[145][146][147]}

The Department of Natural Resources signed a contract with PanoAI for the installation of 360 degree 'rapid detection' cameras around the Pacific northwest, which are mounted on cell towers and are capable of 24/7 monitoring of a 15 mile radius.^[148] Additionally, Sensaio Tech, based in Brazil and Toronto, has released a sensor device that continuously monitors 14 different variables common in forests, ranging from soil temperature to salinity. This information is connected live back to clients through dashboard visualizations, while mobile notifications are provided regarding dangerous levels.^[149]

Satellite and aerial monitoring

Satellite and aerial monitoring through the use of planes, helicopter, or UAVs can provide a wider view and may be sufficient to monitor very large, low risk areas. These more sophisticated systems employ GPS and aircraft-mounted infrared or high-resolution visible cameras to identify and target wildfires.^[150][151] Satellite-mounted sensors such as Envisat's Advanced Along Track Scanning Radiometer and European Remote-Sensing Satellite's Along-Track Scanning Radiometer can measure infrared radiation emitted by fires, identifying hot spots greater than 39 °C (102 °F).^[152][153] The National Oceanic and Atmospheric Administration's Hazard Mapping System combines remote-sensing data from satellite sources such as Geostationary Operational Environmental Satellite (GOES), Moderate-Resolution Imaging Spectroradiometer (MODIS), and Advanced Very High Resolution Radiometer (AVHRR) for detection of fire and smoke plume locations.^[154][155] However, satellite detection is prone to offset errors, anywhere from 2 to 3 kilometers (1 to 2 mi) for MODIS and AVHRR data and up to 12 kilometers (7.5 mi) for GOES data.^[156] Satellites in geostationary orbits may become disabled, and satellites in polar orbits are often limited by their short window of observation time. Cloud cover and image resolution may also limit the effectiveness of satellite imagery.^[157] Global Forest Watch^[158] provides detailed daily updates on fire alerts.^[159]

In 2015 a new fire detection tool is in operation at the U.S. Department of Agriculture (USDA) Forest Service (USFS) which uses data from the Suomi National Polar-orbiting Partnership (NPP) satellite to detect smaller fires in more detail than previous space-based products. The high-resolution data is used with a computer model to predict how a fire will change direction based on weather and land conditions.^[160]

In 2014, an international campaign was organized in South Africa's Kruger National Park to validate fire detection products including the new VIIRS active fire data. In advance of that campaign, the Meraka Institute of the Council for Scientific and Industrial Research in Pretoria, South Africa, an early adopter of the VIIRS 375 m fire product, put it to use during several large wildfires in Kruger.^[161]

Since 2021 NASA has provided active fire locations in near real-time via the Fire Information for Resource Management System (FIRMS).

Between 2022–2023, wildfires throughout North America prompted an uptake in the delivery and design of various technologies using artificial intelligence for early detection, prevention, and prediction of wildfires.^{[162][163][164]}

Costs of wildfire suppression

The suppression of wild fires takes up a large amount of a country's gross domestic product which directly affects the country's economy.^[173] While costs vary wildly from year to year, depending on the severity of each fire season, in the United States, local, state, federal and tribal agencies collectively spend tens of billions of dollars annually to suppress wildfires. In the United States, it was reported that approximately $6 billion was spent between 2004–2008 to suppress wildfires in the country.^[173] In California, the U.S. Forest Service spends about $200 million per year to suppress 98% of wildfires and up to $1 billion to suppress the other 2% of fires that escape initial attack and become large.^[174]

Wildland firefighting safety

Wildland fire fighters face several life-threatening hazards including heat stress, fatigue, smoke and dust, as well as the risk of other injuries such as burns, cuts and scrapes, animal bites, and even rhabdomyolysis.^[175][176] Between 2000 and 2016, more than 350 wildland firefighters died on-duty.^[177]

Especially in hot weather conditions, fires present the risk of heat stress, which can entail feeling heat, fatigue, weakness, vertigo, headache, or nausea. Heat stress can progress into heat strain, which entails physiological changes such as increased heart rate and core body temperature. This can lead to heat-related illnesses, such as heat rash, cramps, exhaustion or heat stroke. Various factors can contribute to the risks posed by heat stress, including strenuous work, personal risk factors such as age and fitness, dehydration, sleep deprivation, and burdensome personal protective equipment. Rest, cool water, and occasional breaks are crucial to mitigating the effects of heat stress.^[175]

Smoke, ash, and debris can also pose serious respiratory hazards for wildland firefighters. The smoke and dust from wildfires can contain gases such as carbon monoxide, sulfur dioxide and formaldehyde, as well as particulates such as ash and silica. To reduce smoke exposure, wildfire fighting crews should, whenever possible, rotate firefighters through areas of heavy smoke, avoid downwind firefighting, use equipment rather than people in holding areas, and minimize mop-up. Camps and command posts should also be located upwind of wildfires. Protective clothing and equipment can also help minimize exposure to smoke and ash.^[175]

Firefighters are also at risk of cardiac events including strokes and heart attacks. Firefighters should maintain good physical fitness. Fitness programs, medical screening and examination programs which include stress tests can minimize the risks of firefighting cardiac problems.^[175] Other injury hazards wildland firefighters face include slips, trips, falls, burns, scrapes, and cuts from tools and equipment, being struck by trees, vehicles, or other objects, plant hazards such as thorns and poison ivy, snake and animal bites, vehicle crashes, electrocution from power lines or lightning storms, and unstable building structures.^[175]

Fire retardants

Fire retardants are used to slow wildfires by inhibiting combustion. They are aqueous solutions of ammonium phosphates and ammonium sulfates, as well as thickening agents.^[178] The decision to apply retardant depends on the magnitude, location and intensity of the wildfire. In certain instances, fire retardant may also be applied as a precautionary fire defense measure.^[179]

Typical fire retardants contain the same agents as fertilizers. Fire retardants may also affect water quality through leaching, eutrophication, or misapplication. Fire retardant's effects on drinking water remain inconclusive.^[180] Dilution factors, including water body size, rainfall, and water flow rates lessen the concentration and potency of fire retardant.^[179] Wildfire debris (ash and sediment) clog rivers and reservoirs increasing the risk for floods and erosion that ultimately slow and/or damage water treatment systems.^[180][181] There is continued concern of fire retardant effects on land, water, wildlife habitats, and watershed quality, additional research is needed. However, on the positive side, fire retardant (specifically its nitrogen and phosphorus components) has been shown to have a fertilizing effect on nutrient-deprived soils and thus creates a temporary increase in vegetation.^[179]

Modeling

On the atmosphere

Most of Earth's weather and air pollution resides in the troposphere, the part of the atmosphere that extends from the surface of the planet to a height of about 10 kilometers (6 mi). The vertical lift of a severe thunderstorm or pyrocumulonimbus can be enhanced in the area of a large wildfire, which can propel smoke, soot (black carbon), and other particulate matter as high as the lower stratosphere.^[187] Previously, prevailing scientific theory held that most particles in the stratosphere came from volcanoes, but smoke and other wildfire emissions have been detected from the lower stratosphere.^[188] Pyrocumulus clouds can reach 6,100 meters (20,000 ft) over wildfires.^[189] Satellite observation of smoke plumes from wildfires revealed that the plumes could be traced intact for distances exceeding 1,600 kilometers (1,000 mi).^[190] Computer-aided models such as CALPUFF may help predict the size and direction of wildfire-generated smoke plumes by using atmospheric dispersion modeling.^[191]

Wildfires can affect local atmospheric pollution,^[192] and release carbon in the form of carbon dioxide.^[193] Wildfire emissions contain fine particulate matter which can cause cardiovascular and respiratory problems.^[194] Increased fire byproducts in the troposphere can increase ozone concentrations beyond safe levels.^[195]

On ecosystems

Wildfires are common in climates that are sufficiently moist to allow the growth of vegetation but feature extended dry, hot periods.^[21] Such places include the vegetated areas of Australia and Southeast Asia, the veld in southern Africa, the fynbos in the Western Cape of South Africa, the forested areas of the United States and Canada, and the Mediterranean Basin.

High-severity wildfire creates complex early seral forest habitat (also called "snag forest habitat"), which often has higher species richness and diversity than unburned old forest.^[196] Plant and animal species in most types of North American forests evolved with fire, and many of these species depend on wildfires, and particularly high-severity fires, to reproduce and grow. Fire helps to return nutrients from plant matter back to the soil. The heat from fire is necessary to the germination of certain types of seeds, and the snags (dead trees) and early successional forests created by high-severity fire create habitat conditions that are beneficial to wildlife.^[196] Early successional forests created by high-severity fire support some of the highest levels of native biodiversity found in temperate conifer forests.^[197][198] Post-fire logging has no ecological benefits and many negative impacts; the same is often true for post-fire seeding.^[129] The exclusion of wildfires can contribute to vegetation regime shifts, such as woody plant encroachment.^[199][200]

Although some ecosystems rely on naturally occurring fires to regulate growth, some ecosystems suffer from too much fire, such as the chaparral in southern California and lower-elevation deserts in the American Southwest. The increased fire frequency in these ordinarily fire-dependent areas has upset natural cycles, damaged native plant communities, and encouraged the growth of non-native weeds.^{[201][202][203][204]} Invasive species, such as Lygodium microphyllum and Bromus tectorum, can grow rapidly in areas that were damaged by fires. Because they are highly flammable, they can increase the future risk of fire, creating a positive feedback loop that increases fire frequency and further alters native vegetation communities.^[42][117]

In the Amazon rainforest, drought, logging, cattle ranching practices, and slash-and-burn agriculture damage fire-resistant forests and promote the growth of flammable brush, creating a cycle that encourages more burning.^[205] Fires in the rainforest threaten its collection of diverse species and produce large amounts of CO₂.^[206] Also, fires in the rainforest, along with drought and human involvement, could damage or destroy more than half of the Amazon rainforest by 2030.^[207] Wildfires generate ash, reduce the availability of organic nutrients, and cause an increase in water runoff, eroding other nutrients and creating flash flood conditions.^[36][208] A 2003 wildfire in the North Yorkshire Moors burned off 2.5 square kilometers (600 acres) of heather and the underlying peat layers. Afterwards, wind erosion stripped the ash and the exposed soil, revealing archaeological remains dating to 10,000 BC.^[209] Wildfires can also have an effect on climate change, increasing the amount of carbon released into the atmosphere and inhibiting vegetation growth, which affects overall carbon uptake by plants.^[210]

On waterways

Debris and chemical runoff into waterways after wildfires can make drinking water sources unsafe.^[211] Though it is challenging to quantify the impacts of wildfires on surface water quality, research suggests that the concentration of many pollutants increases post-fire. The impacts occur during active burning and up to years later.^[212] Increases in nutrients and total suspended sediments can happen within a year while heavy metal concentrations may peak 1–2 years after a wildfire.^[213]

Benzene is one of many chemicals that have been found in drinking water systems after wildfires. Benzene can permeate certain plastic pipes and thus require long times to be removed from the water distribution infrastructure. Researchers estimated that, in worst case scenarios, more than 286 days of constant flushing of a contaminated HDPE service line were needed to reduce benzene below safe drinking water limits.^[214][215] Temperature increases caused by fires, including wildfires, can cause plastic water pipes to generate toxic chemicals^[216] such as benzene.^[217]

On plant and animals

Smoke trail from a fire seen while looking towards Dargo from Swifts Creek, Victoria, Australia, 11 January 2007

Airborne hazards

The most noticeable adverse effect of wildfires is the destruction of property. However, hazardous chemicals released also significantly impact human health.^[228]

Wildfire smoke is composed primarily of carbon dioxide and water vapor. Other common components present in lower concentrations are carbon monoxide, formaldehyde, acrolein, polyaromatic hydrocarbons, and benzene.^[229] Small airborne particulates (in solid form or liquid droplets) are also present in smoke and ash debris. 80–90% of wildfire smoke, by mass, is within the fine particle size class of 2.5 micrometers in diameter or smaller.^[230]

Carbon dioxide in smoke poses a low health risk due to its low toxicity. Rather, carbon monoxide and fine particulate matter, particularly 2.5 μm in diameter and smaller, have been identified as the major health threats.^[229] High levels of heavy metals, including lead, arsenic, cadmium, and copper were found in the ash debris following the 2007 Californian wildfires. A national clean-up campaign was organised in fear of the health effects from exposure.^[231] In the devastating California Camp Fire (2018) that killed 85 people, lead levels increased by around 50 times in the hours following the fire at a site nearby (Chico). Zinc concentration also increased significantly in Modesto, 150 miles away. Heavy metals such as manganese and calcium were found in numerous California fires as well.^[232] Other chemicals are considered to be significant hazards but are found in concentrations that are too low to cause detectable health effects.^{[citation needed]}

The degree of wildfire smoke exposure to an individual is dependent on the length, severity, duration, and proximity of the fire. People are exposed directly to smoke via the respiratory tract through inhalation of air pollutants. Indirectly, communities are exposed to wildfire debris that can contaminate soil and water supplies.

The U.S. Environmental Protection Agency (EPA) developed the air quality index (AQI), a public resource that provides national air quality standard concentrations for common air pollutants. The public can use it to determine their exposure to hazardous air pollutants based on visibility range.^[233]

Health effects

Wildfire smoke contains particulates that may have adverse effects upon the human respiratory system. Evidence of the health effects should be relayed to the public so that exposure may be limited. The evidence can also be used to influence policy to promote positive health outcomes.^[234]

Inhalation of smoke from a wildfire can be a health hazard.^[235] Wildfire smoke is composed of combustion products i.e. carbon dioxide, carbon monoxide, water vapor, particulate matter, organic chemicals, nitrogen oxides and other compounds. The principal health concern is the inhalation of particulate matter and carbon monoxide.^[236]

Particulate matter (PM) is a type of air pollution made up of particles of dust and liquid droplets. They are characterized into three categories based on particle diameter: coarse PM, fine PM, and ultrafine PM. Coarse particles are between 2.5 micrometers and 10 micrometers, fine particles measure 0.1 to 2.5 micrometers, and ultrafine particle are less than 0.1 micrometer. lmpact on the body upon inhalation varies by size. Coarse PM is filtered by the upper airways and can accumulate and cause pulmonary inflammation. This can result in eye and sinus irritation as well as sore throat and coughing.^[237][238] Coarse PM is often composed of heavier and more toxic materials that lead to short-term effects with stronger impact.^[238]

Smaller PM moves further into the respiratory system creating issues deep into the lungs and the bloodstream.^[237][238] In asthma patients, PM_2.5 causes inflammation but also increases oxidative stress in the epithelial cells. These particulates also cause apoptosis and autophagy in lung epithelial cells. Both processes damage the cells and impact cell function. This damage impacts those with respiratory conditions such as asthma where the lung tissues and function are already compromised.^[238] Particulates less than 0.1 micrometer are called ultrafine particle (UFP). It is a major component of wildfire smoke.^[239] UFP can enter the bloodstream like PM_2.5–0.1 however studies show that it works into the blood much quicker. The inflammation and epithelial damage done by UFP has also shown to be much more severe.^[238] PM_2.5 is of the largest concern in regards to wildfire.^[234] This is particularly hazardous to the very young, elderly and those with chronic conditions such as asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis and cardiovascular conditions. The illnesses most commonly associated with exposure to fine PM from wildfire smoke are bronchitis, exacerbation of asthma or COPD, and pneumonia. Symptoms of these complications include wheezing and shortness of breath and cardiovascular symptoms include chest pain, rapid heart rate and fatigue.^[237]

Epidemiology

The examples and perspective in this section deal primarily with United States and do not represent a worldwide view of the subject. (July 2023)

The Western US has seen an increase in both the frequency and intensity of wildfires over the last several decades. This has been attributed to the arid climate of there and the effects of global warming. An estimated 46 million people were exposed to wildfire smoke from 2004 to 2009 in the Western US. Evidence has demonstrated that wildfire smoke can increase levels of airborne particulate.^[234]

The EPA has defined acceptable concentrations of PM in the air, through the National Ambient Air Quality Standards and monitoring of ambient air quality has been mandated.^[254] Due to these monitoring programs and the incidence of several large wildfires near populated areas, epidemiological studies have been conducted and demonstrate an association between human health effects and an increase in fine particulate matter due to wildfire smoke.

An increase in PM smoke emitted from the Hayman fire in Colorado in June 2002, was associated with an increase in respiratory symptoms in patients with COPD.^[255] Looking at the wildfires in Southern California in 2003, investigators have shown an increase in hospital admissions due to asthma symptoms while being exposed to peak concentrations of PM in smoke.^[256] Another epidemiological study found a 7.2% (95% confidence interval: 0.25%, 15%) increase in risk of respiratory related hospital admissions during smoke wave days with high wildfire-specific particulate matter 2.5 compared to matched non-smoke-wave days.^[234]

Children participating in the Children's Health Study were also found to have an increase in eye and respiratory symptoms, medication use and physician visits.^[257] Mothers who were pregnant during the fires gave birth to babies with a slightly reduced average birth weight compared to those who were not exposed. Suggesting that pregnant women may also be at greater risk to adverse effects from wildfire.^[258] Worldwide, it is estimated that 339,000 people die due to the effects of wildfire smoke each year.^[259]

Besides the size of PM, their chemical composition should also be considered. Antecedent studies have demonstrated that the chemical composition of PM_2.5 from wildfire smoke can yield different estimates of human health outcomes as compared to other sources of smoke such as solid fuels.^[234]

Post-fire risks

After a wildfire, hazards remain. Residents returning to their homes may be at risk from falling fire-weakened trees. Humans and pets may also be harmed by falling into ash pits. The Intergovernmental Panel on Climate Change (IPCC) also reports that wildfires cause significant damage to electric systems, especially in dry regions.^[260]

Chemically contaminated drinking water, at levels of hazardous waste concern, is a growing problem. In particular, hazardous waste scale chemical contamination of buried water systems was first discovered in the U.S. in 2017,^[261] and has since been increasingly documented in Hawaii, Colorado, and Oregon after wildfires.^[262] In 2021, Canadian authorities adapted their post-fire public safety investigation approaches in British Columbia to screen for this risk, but have not found it as of 2023. Another challenge is that private drinking wells and the plumbing within a building can also become chemically contaminated and unsafe.^[263] Households experience a wide-variety of significant economic and health impacts related to this contaminated water.^[264] Evidence-based guidance on how to inspect and test wildfire impacted wells ^[265] and building water systems was developed for the first time in 2020.^[266] In Paradise, California, for example,^[267] the 2018 Camp Fire caused more than $150 million dollars worth of damage. This required almost a year of time to decontaminate and repair the municipal drinking water system from wildfire damage.

The source of this contamination was first proposed after the 2018 Camp Fire in California as originating from thermally degraded plastics in water systems, smoke and vapors entering depressurized plumbing, and contaminated water in buildings being sucked into the municipal water system. In 2020, it was first shown that thermal degradation of plastic drinking water materials was one potential contamination source.^[268] In 2023, the second theory was confirmed where contamination could be sucked into pipes that lost water pressure.^[269]

Other post-fire risks, can increase if other extreme weather follows. For example, wildfires make soil less able to absorb precipitation, so heavy rainfall can result in more severe flooding and damages like mud slides.^[270][271]

At-risk groups

Residents

Residents in communities surrounding wildfires are exposed to lower concentrations of chemicals, but they are at a greater risk for indirect exposure through water or soil contamination. Exposure to residents is greatly dependent on individual susceptibility. Vulnerable persons such as children (ages 0–4), the elderly (ages 65 and older), smokers, and pregnant women are at an increased risk due to their already compromised body systems, even when the exposures are present at low chemical concentrations and for relatively short exposure periods.^[229] They are also at risk for future wildfires and may move away to areas they consider less risky.^[276]

Wildfires affect large numbers of people in Western Canada and the United States. In California alone, more than 350,000 people live in towns and cities in "very high fire hazard severity zones".^[277]

Direct risks to building residents in fire-prone areas can be moderated through design choices such as choosing fire-resistant vegetation, maintaining landscaping to avoid debris accumulation and to create firebreaks, and by selecting fire-retardant roofing materials. Potential compounding issues with poor air quality and heat during warmer months may be addressed with MERV 11 or higher outdoor air filtration in building ventilation systems, mechanical cooling, and a provision of a refuge area with additional air cleaning and cooling, if needed.^[278]

Human involvement

The human use of fire for agricultural and hunting purposes during the Paleolithic and Mesolithic ages altered pre-existing landscapes and fire regimes. Woodlands were gradually replaced by smaller vegetation that facilitated travel, hunting, seed-gathering and planting.^[288] In recorded human history, minor allusions to wildfires were mentioned in the Bible and by classical writers such as Homer. However, while ancient Hebrew, Greek, and Roman writers were aware of fires, they were not very interested in the uncultivated lands where wildfires occurred.^[289][290] Wildfires were used in battles throughout human history as early thermal weapons. From the Middle Ages, accounts were written of occupational burning as well as customs and laws that governed the use of fire. In Germany, regular burning was documented in 1290 in the Odenwald and in 1344 in the Black Forest.^[291] In the 14th century Sardinia, firebreaks were used for wildfire protection. In Spain during the 1550s, sheep husbandry was discouraged in certain provinces by Philip II due to the harmful effects of fires used in transhumance.^[289][290] As early as the 17th century, Native Americans were observed using fire for many purposes including cultivation, signaling, and warfare. Scottish botanist David Douglas noted the native use of fire for tobacco cultivation, to encourage deer into smaller areas for hunting purposes, and to improve foraging for honey and grasshoppers. Charcoal found in sedimentary deposits off the Pacific coast of Central America suggests that more burning occurred in the 50 years before the Spanish colonization of the Americas than after the colonization.^[292] In the post-World War II Baltic region, socio-economic changes led more stringent air quality standards and bans on fires that eliminated traditional burning practices.^[291] In the mid-19th century, explorers from HMS Beagle observed Australian Aborigines using fire for ground clearing, hunting, and regeneration of plant food in a method later named fire-stick farming.^[293] Such careful use of fire has been employed for centuries in lands protected by Kakadu National Park to encourage biodiversity.^[294]

Wildfires typically occur during periods of increased temperature and drought. An increase in fire-related debris flow in alluvial fans of northeastern Yellowstone National Park was linked to the period between AD 1050 and 1200, coinciding with the Medieval Warm Period.^[295] However, human influence caused an increase in fire frequency. Dendrochronological fire scar data and charcoal layer data in Finland suggests that, while many fires occurred during severe drought conditions, an increase in the number of fires during 850 BC and 1660 AD can be attributed to human influence.^[296] Charcoal evidence from the Americas suggested a general decrease in wildfires between 1 AD and 1750 compared to previous years. However, a period of increased fire frequency between 1750 and 1870 was suggested by charcoal data from North America and Asia, attributed to human population growth and influences such as land clearing practices. This period was followed by an overall decrease in burning in the 20th century, linked to the expansion of agriculture, increased livestock grazing, and fire prevention efforts.^[297] A meta-analysis found that 17 times more land burned annually in California before 1800 compared to recent decades (1,800,000 hectares/year compared to 102,000 hectares/year).^[298]

According to a paper published in the journal Science, the number of natural and human-caused fires decreased by 24.3% between 1998 and 2015. Researchers explain this as a transition from nomadism to settled lifestyle and intensification of agriculture that lead to a drop in the use of fire for land clearing.^[299][300]

Increases of certain tree species (i.e. conifers) over others (i.e. deciduous trees) can increase wildfire risk, especially if these trees are also planted in monocultures.^[301][302] Some invasive species, moved in by humans (i.e., for the pulp and paper industry) have in some cases also increased the intensity of wildfires. Examples include species such as Eucalyptus in California^[303][304] and gamba grass in Australia.