Forensic Fire investigations

Question 1

What is fire?

Answer 1

rapid oxidation of a material in the exothermic chemical process of combustion
-releasing heat, light, reaction products
series of oxidation reactions
-elements and molecules from matter can react with other to make new combinations or be broken down

Question 2

What is combustion?

Answer 2

low molecular weight, highly volatile hydrocarbons can evaporate
combine with O2 in flame
other fuels broken down into smaller components that undergo combustion

Question 3

What is the flash point?

Answer 3

lowest temp at which a volatile or flammable liquid can vapourize to form an ignitable mixture in air
the lower the flashpoint, easier for ignition

Question 4

What is vapour pressure?

Answer 4

measure of volatility of liquid
(tendency to evaporate)
higher the temp, the greater the vapour pressure

Question 5

Induction period and ignition

Answer 5

begins with non-flaming induction period
-may involve smouldering
after an indefinite period, ignition may occur
-transform to flaming combustion
-well ventilated fire

Question 6

What is pyromania?

Answer 6

medical condition whereby people feel compelled to start fires because of anger etc

Question 7

What is arson?

Answer 7

intentional act of setting fire to a property, vehicle etc

Question 8

What does the combustion of pure hydrocarbons form?

Answer 8

water
CO2
heat
CO (if low 02 levels)

Question 9

Give 4 examples of fuels?

Answer 9

petrol
paraffin
alcohol
plastic

Question 10

What are cellulose/carbohydrate based fuels important in?

Answer 10

structural fires

Question 11

Name a solid fuel.

Answer 11

wood

Question 12

Name a liquid fuel.

Answer 12

petrol

Question 13

Name a gaseous fuel.

Answer 13

butane

Question 14

Oxidisers

Answer 14

21% O2 in air
at room temp about 14% O2 will support combustion
at higher temps, lower O2 conc can still support combustion

Question 15

What is the fire point?

Answer 15

temp at which a fuel will continue to burn after ignition
fire point is self-sustaining
usually few degrees higher than flash point

Question 16

What is conduction?

Answer 16

energy transferred by direct contact

Question 17

What is convection?

Answer 17

transferred by mass motion of molecules

Question 18

What is radiation?

Answer 18

transferred by electromagnetic radiation

Question 19

What is the incipent phase?

Answer 19

fire small compared to enclosure

ample air supply to support combustion-well ventilated fire

temp close to normal

plume of hot gas rise from any flame
-soot particles, water vapour, CO2, sulphur dioxide, isocynates

Convection lead to heat and flames to be carried upwards
-O2 drawn at bottom-supports combustion

flame spreads vertically

Question 20

Well ventilated fire

Answer 20

fire grows as more fuel consumed

flames + heat spread upwards due to convection + radiation

fire may spread laterally due to radiation from flames igniting other fuels nearby

hot gasses accumulate in a layer at the top of the room.

top of room is O2 depleted and very hot, smokey and full of toxic gases

bottom of room is relatively cool and has a higher O2 conc

Question 21

Under-ventilated fire

Answer 21

hot gases at top contain potential fuels
-layer deepens as fire continues to burn

radiation or direct contact with flames causes fuels in the smoke layer to ignite

radiated heat from top smoke layer causes all other combustible material in the room to ignite

after fire growth - quasi-steady state is reached

fire continuing in this state until one of two parameters are consumed

fire starts to decay and then smoulder

Question 22

What is smouldering?

Answer 22

when the fuel is exhausted

open flame burning become less and less prevalent

several toxic products are still being produced

supplies of fresh air can cause explosive ignitions

Question 23

Which type of combustion happens when there is a good supply of O2?

Answer 23

complete combustion

Question 24

What is the equation for complete combustion?

Answer 24

methane + O2 –> CO2 + H20

Question 25

When does incomplete combustion happen?

Answer 25

when the fire does not have enough fuel to burn completely

Question 26

What does incomplete combustion form?

Answer 26

carbon monoxide, carbon(soot), CO2 and H20

Question 27

What are the fire hazards?

Answer 27

heat
smoke
toxic combustion products
all factors will affect escape time

Question 28

What is the escape time?

Answer 28

interval between the time of ignition and the time when the conditions are untenable

Question 29

Smoke

Answer 29

gasses or soot

observes vision and inhibits breathing

the higher the smoke developed index (SDI) value of a substance has, the greater amount of smoke it produces

measure of conc of smoke emitted as a substance burns

Question 30

What are asphyxaints?

Answer 30

gases that reduce or displace the normal O2 conc in breathing air

• carbon monoxide
• hydrogen cyanide
• CO2

Question 31

Name 4 irritants that cause irritation to eyes and respiratory tract.

Answer 31

inorganic gases
acrolein
formaldehyde
isocyanates

Question 32

What is asphyxia?

Answer 32

cessation of breathing

causes tissue hypoxia

Question 33

Irritant gases in fires

Answer 33

combustion generates irritant gases

can be subdividded into inorganic acid gases and or organic irritants

inorganic acids generally come from flame retardants

Question 34

Particles

Answer 34

particulates occur naturally, originating from forest fires and volcanoes

also generated by burning fossils

the size of the particle is a main determinant of where in the respiratory tract the particle will deposit.

• larger particles filtered in nose and throat
• > 10 micrometres can settle in bronchi + lungs

Question 35

Who is included in the fire and rescue service?

Answer 35

fire fighters
fire investigators
dog handlers

Question 36

Who is included in the police service?

Answer 36

uniform police officers
CID police officers
scenes of crime officers

Question 37

Fire investigation

Answer 37

locate starting point (v patterns)

fire accidental or deliberatley

identify starting agent e.g.petrol

Question 38

What does fire result?

Answer 38

burn damage
blackened soot residue
toxic fumes

Question 39

Level 1 structural fires and their investigation

Answer 39

routine (FDIR) carried out by operational fire service personnel
the FDIR form is completed by fire-fighters in most structural and vehicle fires

Question 40

Level 2 structural fires and their investigation

Answer 40

more complex investigations led by fire service support personnel

Question 41

Level 3 structural fires and their investigation

Answer 41

complex inter-agency investigations, usually led by the police

Question 42

Fire scene investigations

Answer 42

interview witnesses and fire investigations
examine exterior
examine interior

Question 43

The exterior examination

Answer 43

confirm any witness statements
pattern of burning/damage
locate anything unusual/suspicious around scene

Question 44

The interior examination

Answer 44

work from area of least damage
try to map out progress of fire
-v patterns
-extent/depthnof charring

Question 45

Arson

Answer 45

no accidental cause identified

fire started in improbable location

• centre of room
• adjacent to doorway

multiple points of origin

unusual fire behaviour

evidence of flammable liquids
-hard edged or pool edged low level burns might indicate accelerants

signs of forced entry

fire protection devices/alarms off or tampered

evidence of timers

vandalism, threats

valuables removed

Question 46

What is a physical evidence of accelerants?

Answer 46

burn patterns on floors

Question 47

What are the chemical tests for detecting accelerants?

Answer 47

draegar tubes for collecting gaseous hydrocarbons

fields test kits for hydrocarbons in soil + atmosphere

Question 48

What is the problem with using chemical tests to detect accelerants?

Answer 48

might not be able to discriminate accelerants from pyrolysis products
-acetone and methylated alcohols

tests might not be quantitative or be able to distinguish one accelerant from another.

Question 49

Collecting fire samples

Answer 49

most accelerants consumed in fire
-examine areas which are protected from fire

locate seat of fire and slowly excavate area

work down to unburnt material and areas where accelerant may have seaped

a fist full of general debris required or specific item

packaged in a nylon bag:

• use nylon 11 (rislan bags) (not plastic)
• swan neck and exhibit label
• no adhesive tape to be used -string or ties only
• liquid samples should be placed in a metal accelerant tin and then nylon
• sharps should be placed in an ​appropriate container and then nylon.