REVIEW OF POTENTIAL AIR EMISSIONS FROM BURNING POLYETHYLENE 

PLASTIC SHEETING WITH PILED FOREST DEBRIS

FINAL REPORT

October 28, 2003 

Prepared by: 

Christopher Wrobel 

Tim Reinhardt 

URS Corporation 

1501 4th Avenue, Suite 1400 

Seattle, WA 98101 

Prepared for: 

USDA Forest Service 

Pacific Northwest Research Station 

400 North 34th Street, Suite 201 

Seattle, WA 98103 
1

TABLE OF CONTENTS

Page 

1.0 INTRODUCTION ........................................................................................................ 3 

2.0 BACKGROUND.......................................................................................................... 4 

3.0 CHEMISTRY OF POLYETHYLENE PYROLYSIS AND COMBUSTION MECHANISMS ...... 5 

4.0 CHEMICAL COMPOUNDS EMITTED FROM THE PYROLYSIS AND COMBUSTION OF 

POLYETHYLENE ....................................................................................................... 7 

5.0 BIOMASS EMISSIONS.............................................................................................. 12 

5.1 MECHANISMS OF WOOD PYROLYSIS AND COMBUSTION....................................... 12 

5.2 Emissions From Wood Combustion ..................................................................... 14 

6.0 DISCUSSION ........................................................................................................... 21 

7.0 CONCLUSION.......................................................................................................... 23 

8.0 REFERENCES .......................................................................................................... 25 
1

Executive Summary

URS reviewed the available scientific literature regarding emissions from burning 

polyethylene (PE), and burning silvicultural piles of woody debris. Few papers report 

measuring emissions from polyethylene burned in an open pile, and none has assessed 

emissions from silvicultural piles with and without a PE covering. Thus the literature 

review must draw from bench-scale studies of PE pyrolysis and combustion. These are 

unlikely to duplicate the actual emissions from operational burns of silvicultural piles. 

Bench-scale research investigating the pyrolysis and combustion chemistry and emissions 

from pure PE has identified a broad range of emissions, many of which are also identified 

in emissions from burning woody silvicultural debris. Carbon dioxide (CO2), carbon 

monoxide (CO), water and particulate matter are major emissions from both sources. 

Polycyclic aromatic hydrocarbons (PAHs) are found in emissions from both materials, 

and at widely varying concentrations. Many partially oxygenated products of incomplete 

combustion (PICs) have been quantified in woody debris emissions, and identified but 

not quantified in PE emissions—these along with PAHs and other PICs are known to 

have adverse effects on human and nonhuman receptors when they are exposed at 

significant concentrations. There is no evidence that unique classes of chemicals are, or 

should be found in emissions from burning PE, in comparison to burning wood debris. 

The relative impacts from PE and silvicultural debris cannot be accurately estimated from 

these diverse data sources, but the expected ranges can be summarized. The literature 

suggests that the emissions to the atmosphere contributed by the sheet of PE covering are 

chemically similar to the emissions from the underlying pile of silvicultural debris. For 

many of these emissions, such as CO, CO2 and particulate matter, the amount emitted 

from the woody debris will of course overwhelm the contribution from the PE. Some 

studies indicate that PE does not produce a great deal of PAHs, while others indicate that 

it may be an efficient producer of PAHs, and in theory could produce almost as much 

PAH as the silvicultural pile, but only when two unlikely assumptions are made: 1) that 

PE emissions from pellets in high-temperature furnaces replicate the emissions of PE 

covering a silvicultural pile where temperatures increase more slowly and half or more of 

the PE is likely to be volatilized or burned at lower temperatures before it reaches the 

high temperatures required to form PAHs, and 2) that the relatively complete spectrum of 

individual PAHs identified in PE combustion studies is directly comparable to the smaller 

subset of PAHs monitored from burning woody debris. 

The literature, and anecdotal evidence, clearly indicates that silvicultural piles burn more 

efficiently and produce fewer PICs when they are allowed to cure to a dryness that 

readily supports combustion. Inasmuch as regions in Oregon where silvicultural burning 

occurs are exposed to significant amounts of precipitation, there is an overall emissions 

reduction benefit from covering silvicultural piles. Polyethylene does not include 

chlorinated compounds or significant amounts of other chemicals likely to form uniquely 

toxic emissions, nor have these been demonstrated in the literature. As a covering 

material, it is reportedly much less expensive than coated or uncoated kraft paper, an 

alternative product used to cover piles in central and southern California at the request of 
2

local air quality management districts. There are no literature studies of the emissions 

from coated kraft paper, but the cellulose should be similar to woody debris and the waxy 

coating, when melted, should burn very similarly to melted polyethylene. In Oregon, 

some 314 grams of PE are burned on a typical pile covered with 4-mil PE sheeting. If 1 

or 2-mil sheeting were used, the PE emissions would be cut by half or more. In terms of 

overall emissions from the pile and the cover, the benefit of cutting PE emissions by only 

covering the center of the pile may be offset by higher emissions from uncovered wood 

which has attained higher fuel moistures during the curing period. 

All the bench-scale studies suffer from a weakness in extrapolating from very controlled 

combustion conditions to those found in operational burns. A definitive answer as to 

whether emissions from a PE-covered or partially-covered pile are measurably different 

from the uncovered or paper-covered pile can only be obtained from actual open burning 

of the piles. The available literature does not support a contention that burning PE 

sheeting would produce unique chemicals or classes of chemicals that are not also found 

in emissions from burning wood debris. 
3

1.0 INTRODUCTION

Polyethylene (PE) plastic sheeting has long been used by the forest products industry to 

prevent the accumulation of excessive moisture within silvicultural debris piles as they 

cure over a timeframe of several months to a year or more. The curing process dries the 

piled woody debris and enhances the combustion process, making ignition easier and 

producing fewer products of incomplete combustion. Timber harvest residues (branches 

and small woody fuels) and woody debris from silvicultural thinning and fire hazard 

reduction projects are collected, piled and partially covered with PE sheeting. The 

sheeting (typically six to ten foot squares) is draped over a pile and anchored with sticks, 

logs and rocks to prevent removal by the wind. The sheeting protects the piles from rain 

and snow until they cure. These piles are then ignited with a drip torch and burned, 

usually achieving 100% consumption of the piled debris. The PE sheeting is usually left 

on the pile, to burn, along with the woody debris. Most regulatory agencies enforce 

ordinances against the combustion of plastics in silvicultural debris piles because of 

concerns that the emissions may have an adverse affect on air quality. 

This paper reviews the available literature on the pyrolysis and combustion products of 

PE and assesses whether burning the typical amount of PE produces significantly more or 

different emissions than the woody debris piles themselves. This work has been 

undertaken at the request of the Smoke Management Review Committee of the Oregon 

Smoke Management Plan Citizens Advisory Group. The Committee is a multiparty 

association comprised of parties interested in smoke management and forestry issues in 

Oregon, including the USDA-Forest Service, the U.S. Department of the Interior-Bureau 

of Land Management, the Oregon Department of Forestry, the Oregon Department of 

Environmental Quality, Oregon State University Cooperative Extension, the Nature 

Conservancy, the Oregon Forest Industries Council, Lane Regional Air Pollution Control 

Authority, Jackson County, and small woodland owners and public representatives. The 

Committee has posed the following specific questions to the authors: 

• Is there such a thing as pure PE? And, if so, is pure PE readily available 

(commercially) and how would one tell PE from other polymer products on the 

market that are known to contain toxic materials (PVC, styrene, etc.) and heavy 

metals? 

• What is the basic chemistry of PE and how does it combust and react within a pile 

in an ambient environment? 

• How will PE volatilize by itself, and as part of a slash pile? 

• Are there any material safety data sheets that include a combustion test of PE? 

• Is there a difference in PE emissions between a pile that is completely covered 

and one that uses PE in the center to create a dry spot for ignition? 

• Does it make a difference in the emissions if 1-mil or 4-mil PE sheeting is used? 
4

• Is there any benefit to requiring the PE to sit in the environment for a period of 

time (weather) prior to burning? 

• What are the alternatives to PE and what is the chemical makeup of 

“impregnated” paper product? 

• Is the use of PE cost effective compared to alternative products? 

2.0 BACKGROUND

Polyethylene is the most commonly used plastic, worldwide. It is produced by the free 

radical polymerization of ethylene, 1-propene, 1-butene, 1-hexene or 1-octene. There are 

three main types of PE: high-density PE (HDPE), low-density PE (LDPE) and linear lowdensity PE (LLDPE). The majority of PE produced is either HDPE or LDPE. HDPE 

chains are not branched while LDPE contains about 50 branches for every 1000 carbon 

atoms1. 

Commercially manufactured PE is a relatively pure material. Pure PE is translucent; the 

opaque black sheeting used to cover woody debris piles is produced by adding carbon 

black to the polymer. Carbon black, an amorphous form of carbon, does not possess a 

long-range, or macrocrystalline structure. It does exhibit a short-range crystal structure 

that deviates from both the diamond lattice and graphite lattice with respect to the 

interatomic distances and bond angles2. Water, polycyclic aromatic hydrocarbons 

(PAHs), metallic oxides, salts and other contaminants may be present in trace amounts 

within carbon black3. 

Small quantities of chemical additives are often used in order to enhance or attenuate 

some of PE’s characteristics. PE is easily oxidized at temperatures as low as 180 °C. To 

minimize oxidation during processing, either phenol or phosphite based antioxidant 

chemicals are commonly added. Fatty acid amides are added to aid in the formation of 

plastic sheets. Polyethylene glycol esters, glycerol monostearate and ethoxylated 

secondary amines are sometimes added to reduce PE’s propensity to collect a static 

charge4

. The chemical composition and some of the physical properties of pure PE 

pellets are listed in Table 1. 
5

Table 1. Polyethylene – Chemical Composition and Physical Properties 

Volatiles (wt.%) 100 

Ash (wt.%) 0 

Carbon (wt.%) 85.4 – 86.4 

Hydrogen (wt.%) 13.5 - 14.3 

Sulfur (wt.%) 0 - 0.08 

Nitrogen (wt.%) 0 

Oxygen (wt.%) 0 - 0.2 

Chlorine (wt.%) 0 

Heat Value (MJ/kg) 40.5 

Density (g/cm3) 0.910 - 0.940

Degree of Crystallinity (%) 45 - 55 

Melting Point Range ( oC) 105 - 115 

Molecular Weight (Daltons) 10,000 - 50,000

Reference: 4, 5, 6, 9, 

Poly-America is a manufacturer of the LDPE sheeting used in Oregon as a moisture 

barrier on silvicultural debris piles. Although the material safety data sheet provided by 

the company identifies the product as HDPE and linear LLDPE, the manufacturer’s 

website, and subsequent phone conversations with a representative of Poly-America, 

have confirmed that the material is LDPE. It would be difficult, if not impossible to 

differentiate among the three varieties of polyethylene and most other types of plastics by 

a visual examination of the sheet material. 

The density of LDPE ranges from 0.910 to 0.925 grams per cubic centimeter. Guidance 

from the USDA-Forest Service stipulates that silvicultural debris piles are to be covered 

with a sheet of 4-mil polyethylene plastic of 6-foot by 6-foot minimum dimensions. A 

sheet of LDPE with these dimensions would have an approximate mass of 314 grams 

(11.1 ounces). 

3.0 CHEMISTRY OF POLYETHYLENE PYROLYSIS AND COMBUSTION MECHANISMS

Polyethylene combustion is more akin to the combustion of a liquid, rather than a solid. 

PE melts and the liquid pyrolyzes, then burns, producing a visible flame7, 8. The 

combustion of PE, and all polymers (including wood) is a complex process that is still not 

fully researched. Pyrolysis and combustion reactions occur simultaneously in both the 

liquid and gas phases. Volatile components can continue to thermally degrade, oxidize 

and/or react with other chemical species to produce a complex mix of chemicals9, 10. 

The first step in the pyrolysis of PE is random scission of the polymer into smaller 

subunits, which in HDPE produces a wide range of primary radicals; because LDPE 

contains varying proportions of side chains, it can initially produce secondary and tertiary 

radicals as well 1, 11. Low reaction temperatures favor the formation of long-chain 

radicals, while a higher temperature leads to the formation of a greater number of shortchain radicals. Primary radicals can either undergo β-scission to produce ethene, or 
6

intramolecular hydrogen transfer to form more stable secondary radicals. The latter path 

dominates at low temperatures. High temperatures also increase the rate of volatilization 

of longer chains, where they undergo β-scission at a rate much higher than that found in 

the liquid phase12, 13. 

The rate of pyrolysis increases as the extent of the branching increases. LDPE pyrolyzes 

at a faster rate than that of HDPE, primarily as a result of the stability of the radicals 

formed during the initial step. The exposure of PE to ultraviolet radiation has been 

shown to lower its thermal stability. This trend is less pronounced for branched 

polymers, because they already exhibit significantly lower activation energies11. 

The formation of aromatic rings in the PE pyrolysis/combustion gas is believed to occur 

at higher temperatures due to the rapid thermal degradation of the polymer. The cracking 

of heavy olefins produces gaseous compounds of lighter molecular weight, which at 

temperatures above 730 oC, can react to generate aromatic compounds via the Diels Alder 

mechanism13, 14, 15. 

Chromatographic evidence has determined that at temperatures below 750 oC significant 

pyrolytic degradation of PE occurs, however, complete combustion of PE would not 

occur9

. This may provide some insight into the expected emissions from burning PE with 

a silvicultural debris pile. Immediately after pile ignition, especially considering the low 

thermal conductivity of plastics, the combustion temperature would not be intense 

enough to heat the PE to a temperature high enough to initiate combustion. Because PE 

melts and thermally degrades at relatively low temperatures (105 and 180 oC), 

pyrolysates would be formed and emitted before the temperature can rise high enough to 

ensure more complete combustion. At temperatures below 755 oC, as much as 18 to 41 

percent of the mass of PE is lost and volatilized prior to particle ignition16. 

In the context of a burning silvicultural debris pile, PE pyrolysis chemistry would be of 

far greater importance than combustion chemistry. The temperature of the pile surface 

would rise from ambient to about 1000 o

C over roughly one to five minutes. Thus, the 

PE is likely to undergo thermal degradation and melt during the early stage of 

combustion, when the pile temperature is between 250 and 600 o

C. These temperatures 

are not high enough to allow PE combustion. The emissions from this early phase of the 

pile burn would contain a high percentage of aliphatic hydrocarbons and radicals, as well 

as a low percentage of aromatic and polycyclic aromatic hydrocarbons (PAHs). As the 

biomass pyrolysis and oxidation rates accelerate, the temperature of the pile will increase 

high enough to make PE combustion an important process, but by this time most, if not 

all of the PE would have already been pyrolyzed and lost to the atmosphere. The 

temperature of the plume at the time of PE pyrolysis would be lower due to cooling by 

entrained air, precluding the downwind formation of aromatic compounds because there 

is not enough heat to overcome the activation energy of the Diels Alder reaction. 
7

4.0 CHEMICAL COMPOUNDS EMITTED FROM THE PYROLYSIS AND COMBUSTION OF 

POLYETHYLENE

Most of the data regarding the emissions from the pyrolsis/combustion of PE were 

measured at temperatures ranging from 500 to 1000 oC. The experiments were conducted 

in various apparatus’ that could be described as horizontal laminar flow reactors, vertical 

tube-type furnaces or fluidized bed reactors. In many cases the device had a secondary 

combustion chamber and exhaust samples were collected and analyzed from both the 

primary and secondary stages. Only results reported from a primary combustion stage 

are discussed here, as they are more comparable to the conditions under which PE 

sheeting would burn on a silvicultural debris pile. There is one major difference between 

the pile and the furnace conditions; the temperature of the furnace is maintained at a set 

temperature, while the silvicultural debris pile begins at ambient temperature. This 

means that the pyrolysates and combustion products in a furnace at 800 o

C are exposed to 

that temperature immediately upon initiation of the experiment until the samples are 

collected. In contrast, a silvicultural debris pile will exhibit a temperature gradient. 

Compounds emitted from the thermal degradation of PE will not remain at that 

temperature indefinitely. 

The US EPA lists emission factors for some PAHs due to the burning of agricultural 

plastic17. Specifically, Chapter 2.5.2.4 of the widely used document “AP-42” discusses 

the burning of “agricultural plastic film.” The type of plastic is not expressly stated, 

however, agricultural plastic film is almost always black PE. These experimental 

conditions under which these emission factors were calculated are more similar to those 

expected in silvicultural applications than those obtained under furnace conditions. The 

AP-42 emission factors are rated a “C.” This indicates that there is no known statistical 

bias present in the data, however, the data may not represent a random sample 

population. 

AP-42 emission factors are based on the research performed by Linak et al. at the US 

EPA’s Air and Energy Engineering Research Laboratory. The experimental 

methodology used kerosene to initiate the pyrolysis of 1-lb samples of used and unused 

PE. The clean unused PE was balled in a manner similar to the way in which used 

agricultural film is prepared for incineration in the field. The used PE samples contained 

varying quantities of soil, biomass and water18. 

The thermal decomposition and combustion of PE, as with the combustion of any 

carbonaceous fuel, produces carbon monoxide (CO), carbon dioxide (CO2), water (H2O) 

and a plethora of alkanes, alkenes and aromatic compounds9

. At least 140 chemical 

species have been tentatively identified in the emissions from the pyrolysis and 

gasification of PE19. The pyrolysis energy of activation is approximately 260 kilojoules 

(kJ)/mole, and a range of reaction orders between 0.5 and 1.5 has been reported12, 20. 

Some general trends regarding the types and quantities of emission products from the 

combustion of PE have been identified. CO2 emissions are maximized at around 600 to 

700 o

C, while the minimum concentrations tend to occur near 1000 oC5, 21. CO and 
8

particulate matter concentrations exhibit little variability over the 500 to 1000 oC 

temperature range and CO concentrations peak at around 800 o

C. Particulate matter 

emissions are almost identical at both ends of the temperature range (500 and 1000 oC), 

with little variation found at the temperatures between, although the chemical 

mechanisms and compounds responsible for this observation are different. As much as 

70 to 97 percent of the particulate matter formed was found to have a mean aerodynamic 

diameter of 2 micrometers (μm) or less22. 

The proportions of organic chemicals emitted can vary widely with the PE combustion 

temperature, although most chemicals can be found at some concentration at all 

temperatures23. Emissions of benzene and its derivatives are maximized around 700 to 

800 o

C. From 800 to 900 oC, alkenes, naphthalene, fluorene and phenanthrene were the 

predominant organic compounds detected. At a combustion temperature of 

950 o

C, the production of light hydrocarbons was reduced, while PAH concentrations 

increased5

. PAH concentrations were found to increase along with combustion 

temperature5, 19, 14. Of those PAHs analyzed for; naphthalene was found in the largest 

concentrations followed by acenaphthylene, phenanthrene, pyrene, fluoranthene, 

fluorene, biphenyl, anthracene, cyclopenta[c,d]pyrene, benzo[a]pyrene, 

acephenanthrylene, benzo[b]fluoranthene, chrysene, benzo[g,h,i]fluoranthene, 

benzo[g,h,i]perylene, and perylene5. 

Hydrocarbon emissions tend to exhibit a trend opposite to that of PAHs. Hydrocarbon 

emissions peak in the temperature range of 600 to 800 o

C19, 14. The concentration of 

ethylene was the greatest, due to the β-scission of primary radicals. The mole fractions of 

methane, ethane, and ethylene all reached maximums around 700 °C. At temperatures 

greater than 700 °C the concentrations of methane, ethane, and ethene all decrease5. 

The research suggests that synthetic polymers produce as much as one to two orders of 

magnitude more soot than cellulosic material at typical heating rates (10 –100 K/s). Soot 

production has been found to increase with the combustion temperature for all plastic 

polymers. PAH concentrations in the soot vary, however, soot from PE was found to 

have the greatest concentration of benzo[a]pyrene (1.5 mg/g) among the common 

plastics. Soot production was found to be very low in experiments using very high 

heating rates (~10,000 K/s) and combustion temperatures in excess of 1000 o

C24, 22. 

Oxygenated organic compounds have been identified in the combustion gases of PE. The 

species positively identified include acetaldehyde, acrolein, propionaldehyde, acetone, 

methyl ethyl ketone and acetic acid23, 24. 

Emission factors for those compounds quantified during the pyrolysis/combustion of PE 

have been compiled in Tables 2 and 3. The minimum and maximum emission factors 

found in the literature are reported. It is important to note that the emission factors 

reported were determined under controlled laboratory conditions. The procedures 

generally involved dropping pure PE pellets into a furnace maintained at a known 

temperature. Experimental conditions and sources of error should be expected to vary 

among the cited experiments. The overall conditions at which the emission factors were 
9

obtained may provide values that differ from those produced under the uncontrolled, 

open burn conditions experienced in the field. 

Some additional chemical species that have been qualitatively identified are listed in 

Table 4. A large number of organic chemicals have been reported in PE combustion 

emissions, however, only those chemicals confirmed to be emitted from the 

pyrolysis/combustion of PE are listed. 

Tables 2 and 3 also list the anticipated range of each chemical quantified. These values 

were calculated using the minimum and maximum emission factors for each chemical 

compound and assuming a sheet of PE with a mass of 314 g (6-foot by 6-foot by 4-mil) is 

burned. The mass emissions are only intended to provide an estimate of the range and 

magnitude of the emissions produced from the combustion of the PE sheet. As discussed 

previously, the values from among those studies that reported emission rates were used in 

the calculations. The accuracy of the emission rates is not known, and too few studies 

have been conducted to allow for a meaningful statistical analysis of these data. 

Table 2. Emission Factors and Mass Emissions from Polyethylene Covering 

a Typical Silvicultural Pile – Inorganic, Alkane, Alkene, Aromatic and Total 

Suspended Particulate Matter 

Emission Factor Mass Emitted1 

Chemical 

Minimum Maximum Units Minimum Maximum Units Reference

CO 100 175 mg/ga 31 55 g 5 

CO2 400 1500 mg/g 126 471 g 5 

Acetylene ND 2.9 mg/g ND 0.9 g 5 

Methane 0.2 7 mg/g 0.1 2.2 g 5 

Ethane 0.175 0.6 mg/g 0.1 0.2 g 5 

Ethene 1.5 12 mg/g 0.5 3.8 g 5 

Propene 0.4 1.5 mg/g 0.1 0.5 g 5 

Benzene 1.23E-02 4.78E-02 μg/gb 3.86E-03 1.50E-02 mg 17 

Toluene 3.30E-03 4.60E-03 μg/g 1.04E-03 1.44E-03 mg 17 

Ethylbenzene 6.00E-04 1.20E-03 μg/g 1.88E-04 3.77E-04 mg 17 

1-Hexene 1.00E-03 4.30E-03 μg/g 3.14E-04 1.35E-03 mg 17 

TSP (soot) 8 36 mg/g 2.5 11.3 g 5, 22 

ND = not detected 

1 Mass emissions were calculated using the minimum and maximum emission factors for each 

chemical compound and assuming a sheet of PE with a mass of 314 g is burned. 

a

Milligrams of emission per gram of polyethylene 

b

Micrograms of emission per gram of polyethylene 
10

Table 3. Emission Factors and Mass Emissions from Polyethylene Covering a 

Typical Silvicultural Pile – Polycyclic Aromatic Hydrocarbons 

Emission Factors 

(μg/g)a

Mass Emitted1 

Chemical (g) 

Minimum Maximum Minimum Maximum 

Reference

1-(2-Propenyl)naphthalene 6.2E-01 1.2E+02 1.9E-04 3.8E-02 14 

1,1-Diphenylethylene ND 1.5E+01 ND 4.7E-03 14 

1,2:7,8-Dibenzophenanthrene 2.1E+01 1.5E+02 6.6E-03 4.7E-02 14 

1,2-Binaphthalene 1.7E+00 6.1E+02 5.3E-04 1.9E-01 14 

1,6-Dimethylnaphthalene 3.4E+00 1.8E+02 1.1E-03 5.7E-02 14 

1-Ethylidene indene 6.3E+01 1.5E+02 2.0E-02 4.7E-02 14 

1-Ethylnaphthalene 2.0E+00 6.2E+01 6.3E-04 1.9E-02 14 

1H-Phenalene 2.5E+01 7.4E+02 7.9E-03 2.3E-01 14 

1-Methylanthracene ND 2.8E+01 ND 8.8E-03 14 

1-Methylene indene 6.2E+01 1.2E+02 1.9E-02 3.8E-02 14 

1-Methylfluorene 1.7E+01 1.2E+02 5.3E-03 3.8E-02 14 

1-Methylnaphthalene 2.8E+01 1.5E+03 8.8E-03 4.8E-01 14 

1-Phenylnaphthalene 2.8E+01 1.3E+03 8.8E-03 3.9E-01 14 

1-Phenylphenanthrene 4.0E+01 8.4E+02 1.3E-02 2.6E-01 14 

2,2-Binaphthalene 2.1E+00 4.1E+02 6.6E-04 1.3E-01 14 

2,6-Dimethylnaphthalene ND 2.9E+00 ND 9.1E-04 14 

2-Ethenylnaphthalene 7.5E+01 5.1E+03 2.4E-02 1.6E+00 14 

2-Ethylnaphthalene 2.6E+00 9.0E+01 8.2E-04 2.8E-02 14 

2-Methylanthracene 1.2E+00 4.1E+02 3.8E-04 1.3E-01 14 

2-Methylbiphenyl 4.9E-01 1.9E+02 1.5E-04 6.0E-02 14 

2-Methylfluorene 2.9E+01 5.3E+02 9.1E-03 1.7E-01 14 

2-Methylnaphthalene 2.2E+01 1.8E+03 6.9E-03 5.6E-01 14 

2-Methylphenanthrene 1.6E+01 6.2E+02 5.0E-03 1.9E-01 14 

2-Phenylnaphthalene 7.9E+01 2.8E+03 2.5E-02 8.9E-01 14 

3,4-Dihydrocyclopenta[cd]pyrene 2.7E+01 5.1E+02 8.5E-03 1.6E-01 14 

3-Methylbiphenyl 2.5E+00 2.8E+02 7.9E-04 8.8E-02 14 

3-Methylphenathrene 1.7E+01 7.2E+02 5.3E-03 2.3E-01 14 

4H-Cyclopenta[def]phenanthrene 9.6E+00 3.5E+03 3.0E-03 1.1E+00 14 

4-Methylbiphenyl 1.7E+01 5.5E+01 5.3E-03 1.7E-02 14 

9-Ethenylanthracene 3.0E+00 2.8E+02 9.4E-04 8.8E-02 14 

9-Methylanthracene 1.4E+00 2.3E+02 4.4E-04 7.2E-02 14 

9-Methylphenanthrene 2.8E+01 4.7E+02 8.8E-03 1.5E-01 14 

Acenaphthalene 3.1E+02 9.1E+03 9.7E-02 2.9E+00 5, 14 

Acenaphthene 2.0E+01 1.2E+03 6.3E-03 3.7E-01 14 

Acenaphthylene NO DATA 

Acephenanthrylene 1.3E+02 4.7E+03 4.1E-02 1.5E+00 5, 14 

Anthracene 1.3E-03 3.1E+03 4.1E-07 9.6E-01 5, 14, 17 

Benz[a]anthracene 1.4E-02 5.3E-02 4.5E-06 1.7E-05 17 

Benzo[a]anthracene 8.5E+01 1.3E+03 2.7E-02 4.0E-01 14 

Benzo[a]fluorene 6.2E+01 2.2E+03 1.9E-02 6.8E-01 14 

Benzo[a]pyrene 7.5E-03 1.6E+03 2.4E-06 5.1E-01 5, 14, 17 

Benzo[b]chrysene 2.6E+01 1.2E+02 8.2E-03 3.8E-02 14 
Table 3. (Continued)

11

Emission Factors 

(μg/g)a

Mass Emitted1 

Chemical (g) 

Minimum Maximum Minimum Maximum 

Reference

Benzo[b]fluoranthene ND 3.0E+02 ND 9.4E-02 5 

Benzo[b]fluorene 2.2E+01 1.8E+03 6.9E-03 5.6E-01 14 

Benzo[b]pyrene 9.3E-03 3.5E-02 2.9E-06 1.1E-05 17 

Benzo[c]phenanthrene 3.6E+02 2.0E+03 1.1E-01 6.2E-01 14 

Benzo[e]pyrene 9.7E-03 1.2E+03 3.0E-06 3.9E-01 14, 17 

Benzo[g,h,i]perylene 1.5E-02 4.9E-02 4.7E-06 1.6E-05 17 

Benzo[ghi]fluoranthene 4.7E+01 1.3E+03 1.5E-02 4.2E-01 5, 14 

Benzo[ghi]perylene 3.2E+01 7.5E+02 1.0E-02 2.4E-01 5, 14 

Benzo[j]fluoranthene 3.3E+01 2.5E+02 1.0E-02 7.9E-02 14 

Benzo[k]fluoranthene 2.5E-03 4.7E+02 7.9E-07 1.5E-01 14, 17 

Bibenzo[def,mno]chrysene 8.6E+00 3.3E+02 2.7E-03 1.0E-01 14 

Biphenyl 7.9E+01 4.1E+03 2.5E-02 1.3E+00 5, 14 

Chrysene 1.7E-02 2.8E+03 5.4E-06 8.7E-01 5, 14, 17 

Coronene 4.7E-01 1.5E+02 1.5E-04 4.7E-02 14 

Cyclopenta[cd]pyrene 1.7E+02 2.8E+03 5.3E-02 8.7E-01 5, 14 

Dibenz[ah]anthracene NO DATA 

Dibenzopyrene NDb 2.4E+02 ND 7.5E-02 14 

Dihydronaphthalene 4.4E+00 4.5E+01 1.4E-03 1.4E-02 14 

Diphenylethylene 1.8E+01 2.4E+02 5.7E-03 7.5E-02 14 

Diphenylethyne 2.3E+01 9.5E+02 7.2E-03 3.0E-01 14 

Ethylbiphenyl 5.3E+00 3.9E+02 1.7E-03 1.2E-01 14 

Fluoranthene 1.1E-01 8.6E+03 3.4E-05 2.7E+00 5, 14, 17 

Fluorene 1.5E+02 3.7E+03 4.7E-02 1.2E+00 5, 14 

Indene 3.9E+01 6.5E+02 1.2E-02 2.0E-01 14 

Indeno[1,2,3-cd]pyrene 1.1E-02 8.5E+02 3.4E-06 2.7E-01 14, 17 

Methylanthracene 7.9E-01 2.2E+02 2.5E-04 6.9E-02 14 

Methylbiphenyl 2.5E+00 3.3E+02 7.9E-04 1.0E-01 14 

Methylfluorene ND 2.6E+01 ND 8.2E-03 14 

Methylpyrene 1.1E+02 1.2E+03 3.5E-02 3.7E-01 14 

m-Terphenyl 5.5E-01 1.6E+02 1.7E-04 5.0E-02 14 

Naphthalene 6.7E+01 9.0E+03 2.1E-02 2.8E+00 5, 14 

Perylene 9.2E+00 3.5E+02 2.9E-03 1.1E-01 5, 14 

Phenanthrene 2.4E-02 1.8E+04 7.6E-06 5.7E+00 5, 14, 17 

Phenylindene 4.9E+00 3.3E+02 1.5E-03 1.0E-01 14 

Picene ND 1.4E+02 ND 4.4E-02 14 

Pyrene 5.9E-02 1.4E+04 1.8E-05 4.5E+00 5, 14, 17 

Retene 1.9E-02 3.2E-02 5.9E-06 1.0E-05 17 

Total PAH 0.77 39 

ND = not detected 

1 Mass emissions were calculated using the minimum and maximum emission factors for each 

chemical compound and assuming a sheet of PE with a mass of 314 g is burned. 

a

Micrograms of emission per gram of polyethylene 

b

Not Detected 
12

Table 4. Additional Chemical Compounds Identified Qualitatively in Polyethylene 

Pyrolysis/Combustion Emissions 

Chemical Reference 

Acetaldehyde 23 

Acetic acid 23 

Acetone 23 

Acrolein 23 

Methylethylketone 23 

Propionaldehyde 23 

4- and/or 9-Methylphenanthrene 9 

4H-Benz[def]carbazole 9 

4H-Cyclopenta[cd]pyrene 9 

Acenaphthylene 9 

Benzo[a]fluoranthene 9 

Benzothiophene 9 

Dibenz[a,h]anthracene 9 

Dibenzofuran 9 

5.0 BIOMASS EMISSIONS

5.1 MECHANISMS OF WOOD PYROLYSIS AND COMBUSTION

Forest (woody) biomass is mainly comprised of cellulose, hemicellulose and lignin 

polymers that burn via a series of steps: 1) fuel heating and drying, 2) solid fuel pyrolysis 

to volatiles and char, 3) pyrolysis and oxidation of the volatiles and 4) oxidation of the 

char.25 Heat and mass transport processes are closely linked to, and affect the chemical 

reactions that occur; these chemical reactions in turn release or absorb heat and reaction 

products, which then feed back to the heat and mass transport mechanisms and can cause 

overall rates of combustion to increase or decrease. 

After heat transfer drives off excess moisture and the fuel has reached a high enough 

temperature (150-190 °C), cellulosic materials decompose by two alternate pathways.26 

The first pathway is a reduction in the polymer length by bond scission and dominates at 

temperatures below 300 °C. The cellulosic polymer of wood is broken down into shorter 

subunits by bond scission, releasing water, forming free radicals and carbonyls, carboxyl 

and hydroperoxide groups, which then lead to decarboxylation and decarbonylation 

reactions. This first pathway converts the woody substrate into a reactive carbonaceous 

char and generates H2O, CO2 and CO as the major products, along with the free radicals. 

The char is characterized by a high surface area, a high degree of reactivity, and a large 

proportion of aromatic carbon. The aromatic content of the char increases with increasing 

temperature. 
13

The second pathway, operating above 300 °C, breaks apart the remaining substrate 

molecules by transglycosylation, fission and disproportionation reactions. This second 

pathway produces tarry anhydro sugars (e.g., levoglucosan, 1,6-anhydro-ß-Dglucofuranose, and randomly linked oligosaccharides) and low molecular weight 

volatiles. At higher temperatures (300-500 °C), the tar-forming reactions accelerate and 

overtake the production of char and gases. At such temperatures, the heat of evaporation 

of levoglucosan is so endothermic that the pyrolysis process appears to be controlled by 

the rate of heat transfer rather than the chemical reaction kinetics. 

Smoldering and glowing combustion are both characterized by oxidation of the reactive 

char created from the cellulosic substrate. Smoldering combustion operates at relatively 

low temperatures and involves the formation of reactive char by pyrolysis, chemisorption 

of oxygen on the char, production of CO and CO2, and generation of new reactive sites. 

Incompletely oxidized volatile pyrolysis products are released, and smoldering 

combustion is characterized by a high proportion of these products of incomplete 

combustion. Glowing combustion, which burns the char more rapidly and at higher 

temperature, is incandescent and requires more oxygen than smoldering combustion, but 

both are solid-phase combustion processes.26 

Flaming combustion is the result of oxidation of the gaseous volatiles, which are liberated 

in the breakdown of the cellulosic structure. These volatiles then are combusted in the 

gas phase. Flaming combustion is dominated by free radicals rapidly breaking apart 

larger molecules into simpler compounds and atoms. This involves a chain reaction 

mechanism that can be self-sustaining (e.g., regenerating the initiating radicals), or 

branching (generating more radicals). The relatively high rate of heat release created by 

the combustion of volatiles provides the energy for gasification of the substrate and 

propagation of the fire. 

The rate of combustion is strongly influenced by fuel-specific factors, including the fuel 

bulk density, the arrangement (porosity) of the fuelbed (e.g., the deviation from the 

optimum ratio of air spaces to fuel material within the overall fuel array) and the fuel 

moisture content. This last factor is the driving force behind the use of plastic sheeting 

and other materials to cover piled forest biomass. At higher fuel moistures, more of the 

heat from a fire’s exothermic reactions is used to vaporize excess water from the intact 

fuel matrix, resulting in lower temperatures within the fuelbed and enhancing the fuel 

consumption by smoldering combustion. This produces a much higher proportion of 

incompletely combusted products (e.g., CO and particulate matter). Conversely, at lower 

fuel moistures, less of the heat released is used to turn water to steam, and thus more is 

available to enhance gasification and liberation of volatiles. The volatile compounds 

burn in the gaseous phase with more oxygen available than in smoldering combustion, 

resulting in cleaner combustion and production of more completely oxidized combustion 

products (e.g., CO2). 

In summary, the mechanism of wood combustion is in many ways similar to that of 

polyethylene combustion. Glowing and smoldering combustion are wood-specific 

mechanisms that can produce significant products of incomplete combustion, whereas 
14

flaming combustion results in much more complete, and cleaner, emissions. It is very 

difficult to precisely estimate emissions because they are dependent on the relative 

strengths of these various pyrolysis and combustion processes, which in turn depend on 

fuel moisture and pile arrangement. 

5.2 Emissions from Wood Combustion 

Burning forest biomass produces a diverse array of many hundreds of chemical species, 

ranging from gases to liquids and solid particulate matter. Relatively small amounts of 

sulfur oxides (SOx) are formed due to the low sulfur content of forest fuels. Small 

amounts of oxides of nitrogen (NOx) can be produced from endogenous plant nitrogen 

(e.g., chlorophyll), but temperatures are generally too low to dissociate atmospheric N2.

27 

Table 5 lists maximum mixing ratios for nitrogen species and SO2 observed above a 

bench-scale burning system.28

Table 5. Nitrogen and Sulfur Species Measured in Forest Biomass Smoke 

Compound Maximum Mixing 

Ratio (ppm) Combustion Phase 

NOx 100-160 Flaming 

Nitrous oxide 2-19 Flaming 

Ammonia 5 Smoldering 

Hydrogen cyanide 4-30 Smoldering 

Methyl cyanide 2-10 Smoldering 

Sulfur dioxide 20-30 Flaming 

Carbon by far dominates the emissions from forest biomass burning. The major carbonbearing pollutants can be broadly broken down as shown in Table 6.29

Table 6. Percentage of Carbon Emissions by Phase of Combustion 

Carbon contribution (% by weight) Emission species Flaming phase Smoldering phase 

Carbon dioxide 95.8 86.6 

Carbon monoxide 3.0 10.1 

Organic carbon particulate 0.7 2.3 

Elemental carbon particulate 0.2 0.1 

Methane 0.2 0.5 

Nonmethane hydrocarbons 0.1 0.4 

In flaming combustion, more of the fuelbed carbon is converted to CO2 and elemental 

carbon in the particulate matter, while smoldering combustion produces proportionately 

more CO, organic carbon, methane and nonmethane hydrocarbons. Most of the 

particulate matter (82%) is smaller than 1 μm in aerodynamic diameter, and only 8% is 
15

above 5 micrometers.30 Thus, measurements of fine particles less than 2.5 μm in 

diameter (PM2.5) represent the bulk of particulate emissions. The production of CO is 

highly correlated to the production of particulate matter and PM2.5, with a coefficient of 

determination (r2

) between emission factors for these pollutants in the range of 0.8 to 

0.84.31 Combustion efficiency, a measure of how much of the carbon in the fuelbed is 

converted to CO2, can range from less than 60% for smoldering-phase combustion to 

over 95% for flaming-phase combustion. Many of the pollutants emitted from forest fires 

are well-correlated to combustion efficiency, and algorithms have been developed to 

estimate their production.32

Table 7 lists some of the major air toxics measured in emissions from burning forest 

biomass, and algorithms for calculating their production as a function of combustion 

efficiency or a ratio to commonly-measured emissions such as CO or particulate matter.32 

Table 7. Algorithms and Ratios for Estimating Emissions of Air Pollutants from 

Prescribed Burning 

Pollutant Regression or ratio Emission 

Factor Units Mass Emitted1 Units Reliability 

Carbon dioxide η ×1833a 1.7E+03 g/kg 1.0E+06 g ±5% 

Carbon monoxide 961-η×984 2.6E+01 g/kg 1.6E+04 g r2 = 0.95 

Methane 42.7- η×43.2 1.7E+00 g/kg 9.9E+02 g r2 = 0.77 

Nonmethane hydrocarbons 0.76+EF CH4×0.616b 1.8E+00 g/kg 1.1E+03 g r2 = 0.69 

Fine particles (PM2.5) 67.4-η×66.8 1.3E+02 g/kg 7.8E+04 g r2 = 0.74 

Thoracic particles (PM10) 1.18×EF PM2.5 1.5E+02 g/kg 9.2E+04 g ±30% 

Particulate Matter (TSP) 93.3-η×90.5 7.3E+00 g/kg 4.4E+03 g r2 = 0.54 

Formaldehyde (HCHO) 0.0137×EF CO-0.0179 3.4E-01 g/kg 2.0E+02 g ±30% 

Acrolein 0.0029×EF CO+0.0699 1.5E-01 g/kg 8.7E+01 g ±30% 

Acetaldehyde 0.315×EF HCHO 1.1E-01 g/kg 6.4E+01 g ±50% 

1,3-butadiene 0.00213×EF CO 5.6E-02 g/kg 3.3E+01 g ±40% 

Benzene 0.00592×EF CO 1.6E-01 g/kg 9.3E+01 g ±40% 

Toluene 0.00588×EF CO 1.5E-01 g/kg 9.2E+01 g ±40% 

o-Xylene 0.00089×EF CO 2.3E-02 g/kg 1.4E+01 g ±40% 

m,p-Xylene 0.00161×EF CO 4.2E-02 g/kg 2.5E+01 g ±40% 

n-Hexane 0.00017×EF CO 4.5E-03 g/kg 2.7E+00 g ±40% 

Benzo(a)pyrene 13×EF PM 9.5E+01 μg/kg 5.7E+04 μg ±50% 

PAHs 345×EF PM 2.5E+03 μg/kg 1.5E+06 μg ±50% 

1

 Mass emitted from burning one silvicultural debris pile with a diameter and height equal to eight feet 

(approximate mass = 598 kg). 

a

η = combustion efficiency (%) 

b

EF = emission factor 

For woody fuels, piles have relatively high combustion efficiencies, and thus lower 

emissions of products of incomplete combustion, as compared to broadcast-burned slash. 

Table 8 lists emission factors for piled silvicultural debris from the Pacific Northwest.33
16

Table 8. Emission Factors for Major Pollutants from Piled Silvicultural Debris 

Emission Factor (pounds per ton of fuel consumed) Pile Type Phase PM PM10 PM2.5 CO CO2 CH4 NMHC 

Flaming 11.4 7.4 6.6 44 3492 2.4 2.2 

Smoldering 25.0 15.9 14.0 232 3124 17.8 12.2 Tractorpiled Fire-average 20.4 12.4 10.8 153 3271 11.4 8.0 

Flaming 22.6 13.6 11.8 101 3349 9.4 8.2 

Smoldering 44.2 33.2 31.0 232 3022 30 20.2 Cranepiled Fire-average 36.4 25.6 23.4 185 3143 21.7 15.2 

Other measurements of emissions from bench-scale burning of forest biomass have 

identified a variety of PAHs; these are summarized in Table 9, along with an estimated 

mass of emissions from a typical silvicultural debris pile.34 A list of quantified 

oxygenated organic chemicals and other various pollutants from bench-scale studies are 

presented in Tables 10 and 11. The quantities of each chemical species emitted are listed 

in Tables 9, 10 and 11. The mass range of each chemical was calculated from the 

widely-used Consume 2.1 fire consumption and emissions model, assuming a 

composition of 60 percent Douglas fir and 40 percent western hemlock and assuming a 

dome shaped pile eight feet high and eight feet in diameter. A fuel bulk density of 27.5 

lb/ft3 was assumed, along with a fuelbed porosity of 20%. A pile with these 

characteristics was determined to have a mass of 598 kg. 

Table 9. Emission Factors and Mass Emissions – Polycyclic Aromatic 

Hydrocarbons from Fir and Pine Burns 

Emission Factor 

(μg/g) 

Mass Emitted1 

Chemical (g) 

Minimum Maximum Minimum Maximum 

Reference 

1-(2-Propenyl)naphthalene 

1,1-Diphenylethylene 

1,2:7,8-Dibenzophenanthrene 

1,2-Binaphthalene 

1,6-Dimethylnaphthalene 

1-Ethylidene indene 

1-Ethylnaphthalene 

1H-Phenalene 

1-Methylanthracene 

1-Methylene indene 

1-Methylfluorene 

1-Methylnaphthalene 

1-Phenylnaphthalene 

1-Phenylphenanthrene 

2,2-Binaphthalene 

2,6-Dimethylnaphthalene 

NO DATA 
17

Emission Factor 

(μg/g) 

Mass Emitted1 

Chemical (g) 

Minimum Maximum Minimum Maximum 

Reference 

2-Ethenylnaphthalene 

2-Ethylnaphthalene 

2-Methylanthracene 

2-Methylbiphenyl 

2-Methylfluorene 

2-Methylnaphthalene 2.265 2.575 1.35 1.54 34 

2-Methylphenanthrene 

2-Phenylnaphthalene 

3,4-

Dihydrocyclopenta[cd]pyrene 

3-Methylbiphenyl 

3-Methylphenathrene 

4HCyclopenta[def]phenanthrene 

4-Methylbiphenyl 

9-Ethenylanthracene 

9-Methylanthracene 

9-Methylphenanthrene 

Acenaphthalene 

NO DATA 

Acenaphthene 1.868 2.518 1.12 1.51 34 

Acenaphthylene 1.41 2.418 0.84 1.45 34 

Acephenanthrylene NO DATA 

Anthracene 0.429 0.723 0.26 0.43 34 

Benz[a]anthracene 0.114 0.25 0.07 0.15 34 

Benzo[a]anthracene 

Benzo[a]fluorene NO DATA 

Benzo[a]pyrene 0.019 0.694 0.011 0.415 34, 35 

Benzo[b]chrysene NO DATA 

Benzo[b]fluoranthene 0.037 0.056 0.022 0.033 34 

Benzo[b]fluorene 

Benzo[b]pyrene 

Benzo[c]phenanthrene 

NO DATA 

Benzo[e]pyrene 0.024 0.054 0.014 0.032 34 

Benzo[g,h,i]perylene 

Benzo[ghi]fluoranthene NO DATA 

Benzo[ghi]perylene ND 0.002 ND 0.0012 34 

Benzo[j]fluoranthene NO DATA 

Benzo[k]fluoranthene 0.039 0.136 0.023 0.081 34 

Bibenzo[def,mno]chrysene 

Biphenyl NO DATA 

Chrysene 0.1 0.217 0.06 0.13 34 

Coronene 

Cyclopenta[cd]pyrene NO DATA 

Dibenz[ah]anthracene ND ND ND ND 34 

Dibenzopyrene 

Dihydronaphthalene 

NO DATA 
Table 9 (continued) 

18

Diphenylethylene 

Diphenylethyne 

Ethylbiphenyl 

Fluoranthene 1.351 1.766 0.81 1.06 34 

Fluorene 0.68 0.857 0.41 0.51 34 

Indene NO DATA 

Indeno[1,2,3-cd]pyrene ND ND ND ND 34 

Methylanthracene 

Methylbiphenyl 

Methylfluorene 

Methylpyrene 

m-Terphenyl 

NO DATA 

Naphthalene 13.57 16.96 8.11 10.1 34 

Perylene ND ND ND ND 34 

Phenanthrene 2.594 3.938 1.55 2.35 34 

Phenylindene 

Picene NO DATA 

Pyrene 1.066 1.469 0.64 0.88 34 

Retene NO DATA 

Total PAH 15.3 20.7 

ND = not detected 

1

 Mass emitted from burning one silvicultural debris pile with a diameter and height equal to eight feet 

(approximate mass = 598 kg). 

Table 10. Emission Factors of Major Oxygenated Compounds Emitted from the 

Smoldering Combustion of Wood 

Compound 

Emission 

Factor 

(mg/kg) 

Mass Emitted1 

(g) 

Acetic acid 4.40E+03 2.63E+03 

Methanol 4.30E+03 2.57E+03 

Vinyl Acetate 3.00E+03 1.79E+03 

1-Hydroxy-2-propanone 2.00E+03 1.20E+03 

Pyruvic Aldehyde 2.00E+03 1.20E+03 

Formic acid 9.70E+02 5.80E+02 

2-Furaldehyde 7.90E+02 4.72E+02 

3-Oxobutanoic acid, methylester 7.00E+02 4.19E+02 

2-Furanmethanol 2.80E+02 1.67E+02 

2-Methoxy-4-methylphenol 2.60E+02 1.55E+02 

Propanoic acid 2.50E+02 1.50E+02 

2-(5H)-Furanone 2.20E+02 1.32E+02 

2-Cyclopenten-1-one 1.90E+02 1.14E+02 

2-Methoxyphenol 1.80E+02 1.08E+02 

gamma-Butyrolactone 1.70E+02 1.02E+02 

5-Methyl-2-furaldhyde 1.50E+02 8.97E+01 
Table 10 (continued) 

19

Compound 

Emission 

Factor 

(mg/kg) 

Mass Emitted1 

(g) 

Phenol 1.10E+02 6.58E+01 

4-hydroxy-3-methoxybenzaldehyde 7.60E+01 4.54E+01 

2-Methoxy-4-(1-prop-2-enyl)phenol 6.80E+01 4.07E+01 

Crotonic acid 6.80E+01 4.07E+01 

2-Acetylfuran 5.40E+01 3.23E+01 

m- and/or p-Cresol 4.40E+01 2.63E+01 

o-Cresol 4.20E+01 2.51E+01 

Benzoic acid 2.00E+01 1.20E+01 

o-Hydroxybenzaldehyde 1.50E+01 8.97E+00 

Reference: 36 

1

 Mass emitted from burning one silvicultural debris pile with a diameter and height equal to eight 

feet (approximate mass = 598 kg). 

Table 11. Emission Rates and Mass Emissions for Various Pollutants from 

Burning a Silvicultural Pile 

Emission Rate (g/kg) Mass Emitted1 (g) Compound 

Minimum Maximum Minimum Maximum

Reference 

Acetic Acid 1.6 4.4 963 2631 37, 38, 36 

Acetylene 0.2 0.9 120 526 37, 38 

Ethene 1.1 3.3 661 1961 37, 38 

Ethane 0.2 0.2 140 140 37 

Methane 1.2 19.4 739 11601 37, 39, 38, 40, 35 

Carbon monoxide 28.2 359.0 16834 214682 37, 39, 38, 40, 35 

Carbon dioxide 1146 1771 685308 1059058 37, 39, 38, 40, 35 

Formaldehyde ND 2.4 ND 1423 37, 39, 38 

Formic acid 0.7 1.6 425 939 39, 38, 36 

Hydrogen cyanide 0.6 0.6 365 365 38 

Methanol 0.8 4.3 493 2571 39, 38, 36 

Ammonia 0.3 1.5 169 891 39, 38 

Nitric oxide 0.9 2.3 526 1357 37, 39, 38 

Nitrogen dioxide 0.6 0.6 360 360 37 

Sulfur dioxide 1.2 1.2 705 705 37 

PM 6.0 34.2 3588 20452 33, 41, 42, 35, 43 

PM10 6.2 12.8 3708 7656 28 

PM2.5 2.8 17.6 1674 10525 33, 42, 35, 40, 43 

ND = not detected 

1

 Mass emitted from burning one silvicultural debris pile with a diameter and height equal to eight 

feet (approximate mass = 598 kg). 

In summary, the burning of silvicultural debris produces a wide range of air emissions, 

many of which have toxic or reactive properties. The mass of pollutants emitted is quite 
20

large for a typical pile, but piling has been shown to produce fewer emissions per ton of 

silvicultural debris burned, because it optimizes combustion conditions when compared 

to broadcast burning of debris in-situ.44
21

6.0 DISCUSSION

In this section, we specifically answer the questions posed by the Committee. 

Is pure PE readily available (commercially) and how would one tell PE from other 

polymer products on the market that are known to contain toxic materials (PVC, 

styrene, etc.) and heavy metals? 

Pure PE is commercially available; however, most of the PE manufactured will contain 

various types and amounts of additives. The quantities of these chemicals added to the 

PE would be small and should not significantly affect the types or amounts of chemicals 

emitted when PE is burned. The carbon black added to the polymer may contain trace 

amounts of PAHs, metal oxides and other contaminants. Again, this is unlikely to 

produce any detectable increase in emissions as carbon black comprises a few percent of 

the total mass of PE and the contaminants are present in only trace amounts in the carbon 

black itself. One study determined that there was no statistical difference in the heavy 

metal concentrations found in the residual ash of a coal-fired boiler with, or without the 

addition of PE waste to the fuel45. It would be difficult, if not impossible to differentiate 

among the three varieties of polyethylene and most other types of plastics by a visual 

examination of the sheet material. 

What is the basic chemistry of PE and how does it combust and react within a pile in 

an ambient environment? 

How PE will pyrolyze and combust within a slash pile can only be surmised based on the 

available research regarding its behavior when burned in a furnace or alone using 

kerosene as an accelerant. Several trends are evident in the pyrolysis/combustion 

behavior of PE. First, the production of PAHs will increase as the combustion 

temperature increases from 500 to 1000o

C. Second, the emissions of light hydrocarbon 

will peak around 700o

C. Third, particulate emissions are the highest at both low and high 

ends of the temperature range, although the emission factor does not deviate considerably 

throughout the range of temperatures. Third, complete combustion of PE will not occur 

at temperatures below 750o

C. 

How will PE volatilize by itself, and as part of a slash pile? 

Based on the available data, it can be assumed that the volatilization of the PE will 

commence at a relatively low temperature. In the early stage of the pile burn, 

temperatures will quickly rise from ambient to around 350 to 400 oC. This is high 

enough to thermally degrade the PE, but not high enough to allow combustion. The 

pyrolysates will melt and drip into the hotter region of the fire and volatilize into the 

atmosphere. Since up to 41 percent of the PE is lost prior to ignition at temperatures 

lower than 750 o

C, little PE will remain by the time the pile temperature becomes high 

enough to favor the formation of PAHs. This means the emissions of PAHs from the PE 

are likely to be low. 
22

Are there any material safety data sheets that include a combustion test of PE? 

Material Safety Data sheets do not contain detailed information regarding the products of 

incomplete combustion of PE because the raw material is not considered hazardous and 

burning is not an intended use of PE sheeting. The manufacturer provided material safety 

data sheet for the PE sheeting lists CO2 and H2(O) as the compounds emitted during 

combustion and states CO would be produced under oxygen limiting conditions. The 

actual emissions from any fuel will vary considerably with the conditions under which it 

is burned, but are almost never limited to these three chemicals. 

Is there a difference in PE emissions between a pile that is completely covered and one 

that uses PE in the center to create a dry spot for ignition? 

A smaller amount of PE would of course produce a proportionately smaller mass of 

emissions. From the pile as a whole, research has documented that higher fuel moisture 

results in higher overall emissions due to the increased dominance of smoldering 

combustion processes. An uncovered pile that is left over a winter can take up to two 

drip torches of fuel to sustain ignition—this would increase overall emissions due to the 

excess fuel used. However, when piled forest materials are dry enough to sustain 

ignition, the combustion of the smaller-diameter materials in the fuelbed produce enough 

heat to drive off fuel moisture from the larger fuel pieces. This reduces the consumptionlimiting effect of high fuel moisture, increases the combustion efficiency and consumes 

virtually all of the woody material in the pile. 

Does it make a difference in the emissions if 1-mil or 4-mil PE sheeting is used? 

The amount of PE used will proportionately affect on the overall amount of chemicals 

emitted from the PE. Using PE sheeting of only 1-mil thickness would cut the emissions 

by one quarter. If thinner PE is determined to work adequately, its use should be 

considered. Thinner PE would be more cost-effective, and each roll would be easier to 

handle since it would weight less per square foot. However, if the thinner PE sheet is 

easily torn on the fuel pile , it may not be quite as effective at drying the fuels beneath. 

Is there any benefit to requiring the PE to sit in the environment for a period of time 

(weather) prior to burning? 

There is no benefit to allowing the PE sheeting time to weather prior to burning. 

Exposure to ultraviolet radiation will begin to break the polymer down, but this will not 

have a significant affect on the overall combustion efficiency of the burn. 
23

What are the alternatives to PE and what is the chemical makeup of “impregnated” 

kraft paper? 

The only alternative to PE used by the forest products industry is kraft paper. The kraft 

paper is often treated with a waxy hydrocarbon (paraffin) to aid in its ability to resist 

water penetration. Anecdotal reports indicate some problems with the durability of the 

kraft paper, especially in high-precipitation climates. Paraffin is a long-chain 

hydrocarbon similar to some of the n-alkane emissions produced from the pyrolysis of 

PE. Kraft paper is made from cellulose and is therefore similar to the biomass contained 

in the silvicultural debris piles. The cellulose has been liberated from the lignin during 

the pulping process using heat, pressure, sodium hydroxide and sodium sulfide. The 

paper may contain trace amounts of residual sulfur compounds and could produce sulfurcontaining emissions upon combustion. The quantity of sulfur emitted is not expected to 

be large. 

Is the use of PE cost effective compared to alternative products? 

From an economic standpoint, PE is more cost-effective than paper. The paper used to 

cover silvicultural debris piles may or may not be coated with paraffin. The coated paper 

costs about $160 per 3 ft by 200 ft roll, which is 26.7 cents per square foot. PE sheeting 

comes in numerous sizes, although Grayback Forestry in Oregon has indicated that they 

purchase the material in rolls 3 ft wide, weighing 22.9 lbs. A roll 3 ft by 400 ft would 

cost $14.40 (assuming at least 5000 lbs of PE is purchased per order) or about 1.2 cents 

per square foot. 

7.0 CONCLUSION

The benefit obtained from the increased combustion efficiency commensurate with dry 

biomass fuel favors the use of some sort of moisture barrier to aid in the drying of 

logging slash piles. The articles reviewed provide no evidence that burning the PE 

plastic sheeting along with the slash pile would cause a significant impact to air quality, 

however, the limited amount of information regarding the pyrolysis/combustion of PE in 

conjunction with woody biomass precludes an ironclad statement. It is, however, likely 

that the emission factors for PAHs and single ring aromatic compounds obtained for the 

burning of PE in furnace type conditions overestimates the amount that would be formed 

during open burning. This is due to the fact that during an open burn, the pile 

temperature would be much lower than the 900 to 1000 o

C temperatures that favor the 

formation of PAHs. By the time the higher temperatures are reached, most of the PE will 

have thermally degraded and volatilized. 

Because the conditions under which PE pyrolysis and combustion have been studied are 

so varied, it is difficult to quantify the expected emissions of hazardous air pollutants. At 

least two researchers agree on the expected concentration range of TSP (soot) in the 

smoke from PE pyrolysis/combustion (8 to 36 mg per gram of material). However, other 

articles provide ranges for PAH emissions that are about 1000 times greater than those 
24

listed in AP-42. The emission factors for the 77 PAHs found indicate that a sheet of 4 

mil PE weighing 314 grams would produce between 2.5 and 11.3 grams of TSP and 

between 0.8 and 39 grams of PAH compounds when burned. 

The literature is almost as ambiguous concerning biomass emissions as it is for PE. 

Using the lowest and highest emission factors for PM found in the literature gives a range 

of emissions between 3.6 and 20.5 kg per 598 kg pile. Empirical data from Table 8 for 

crane-piled debris (similar to hand piling) would lead to an estimate of 10.9 kg of PM. 

Using the TSP emission factor from Table 8 (converting units--18.2 g/kg) in the equation 

for PAHs listed in Table 7, returns a value of 3.8 grams of PAHs per slash pile. This 

mass of PAHs is considerably lower than the range of 15 to 21 grams per pile calculated 

using the emission factors for the 16 PAHs published by Jenkins et al. Further 

complicating the issue, because no published data exist for prescribed burning emissions 

of most of the 77 PAHs quantified in the PE research, a direct comparison of total PAH 

emissions can not be made between the two sources. 

Unfortunately, the literature has not provided an unequivocal answer to whether burning 

PE plastic would have any greater impact on air quality than the burning of biomass. 

This is surprising given that AP-42 emission factors were determined in response to the 

agricultural industry’s practice of burning PE sheeting material. If this is an ongoing 

practice across the country, then it seems contradictory to allow it for one industry, but 

not another. Unfortunately at this time, the only way to be sure burning PE with logging 

slash would not be detrimental to air quality is to conduct an experimental burn using the 

same type of fuel with and without PE sheeting and to quantify the emissions. 
25

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