Contamination of the scene and evidence

what do we mean by contamination?

• Make (something) impure by exposure to or addition of a poisonous or polluting substance

forensic context of contamination

• The addition, removal or transfer of material from one item, scene or person to another item scene or person

Fibres, DNA

Locard’s Exchange Principle

'any action of an individual and obviously, the violent actions of a crime, cannot occur without leaving a trace'

Every contact leaves a trace

More violent=more likely to happen

When contact made exchange is made

transfer between…

• Victim

• Offender

• Scene

secondary transfer

• Offenders address

• Vehicles-victim's blood, fibres, suspect blood(primary)

• Items handled

affecting factors for transfer of material

1. Pressure-more=more likely to leave trace

2. Number-how many times touched it

3. Time-how long holding it for

4. Texture-latex=less likely to shed that a wool coat

affecting factors of material retention

1. Texture-latex less likely to hold evidence than wool coat

2. Washing-less likely to hold evidence

3. Activity-more activity=more movement=more likely to come off

4. Time-how long holding it for

 

how does contamination occur?

Primary transfer (direct)-no suit into scene leaving trace

Secondary transfer (indirect)- go into scene with suit, touch desk and pick up prints, touch van steering wheel leaving said prints

Poor packaging-want to package so nothing enters the bag

 

Tertiary transfer (indirect)

Inverse transfer (direct)- could take DNA from scene onto us

 

FSR contamination

 introduction of DNA or biological material containing DNA, to an exhibit at or after the point when a controlled forensic process starts

sources of contamination

1. Personnel (to exhibit or DNA sample)

2. Contaminated consumables or packaging materials

3. Contaminated equipment

4. Environmental factors

it is an operational environment

1. Accept risk on contamination

2. Accept environmental risk

3. Identify risk

4. Use measures to reduce risk-double glove and change gloves when handling exhibit with possible DNA

examining the scene

Prior activities-cleaning equipment and van

Discussion with victim-movement around scene

Attendance at scene- initial walkthrough and visual examination

Scene examination- order of examination and recovery

Completion of scene examination- cleaning down the scene

 

minimum PPE volume crime

• Face mask

• Gloves (wipe down)

• Second pair of gloves-when recovering items for DNA

• Consideration further PPE- or alternative barrier clothing should scene require

minimum PPE for serious/major crime

• Face mask

• Mob cap/ hairnet

• First pair of gloves (wipe down)

• Scene suit (hood)

• Overshoes

• 2nd pair of gloves (wipe down)

avoiding contamination

Clean kit and sterile areas

Clean vans and kits

Sterile sheet down and kit on top

photographing prior to recovery

Photograph item in situ prior to moving

For detailed photography use sterile sheet

Items should be handles as little as possible

Use separate sterile sheet for each exhibit for each exhibits and recover the sheet

packaging

All items recovered from a scene must be packaged, sealed and labelled in entirety whilst at scene

Use correct size of packaging to ensure item is completely covered

Item should be handled as little as possible

Use separate packaging for each item where packaging of items together is likely to compromise them

anti-contamination guidance

• Scene preservation and advice to others-victim leave things, FAO cover footprints e.g.

• Use of clean equipment-clean down kit after before scene/after scene

• Methodical scene approach

• Contamination risks between different parts of the same scene- sterile areas- new sheet of sterile paper for each exhibit

• Correct use of barrier clothing/PPE

• Awareness of primary and secondary transfer issues

• Correct packaging, continuity, integrity and security

why is it necessary to clean the scene after examination?

• Health and safety - biohazards left at scene

• Vulnerable victims - don’t want to trigger them

• Future incidents and powder remaining- may be investigated in future don’t want it to affect that

• contamination