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Chaynee Hodgetts

Features and Opinion Editor & Barrister, Solicitors Journal & Libertas Chambers

Quotation Marks
‘“Every contact leaves a trace...” …known in the sciences as “Locard’s Exhange Principle”, this is one of the central tenets of the study of trace evidence in forensics”

Hot stuff?: Fireworks, forensics, & familiarising lawyers with science

Hot stuff?: Fireworks, forensics, & familiarising lawyers with science


Chaynee Hodgetts considers the subject of forensic education for lawyers

“Every contact leaves a trace...” Perhaps better known in the sciences as “Locard’s Exhange Principle”, this is one of the central tenets of the study of trace evidence in forensics (E. Locard, “The Analysis of Dust Traces. Part I.” (1930) 1 American Journal of Political Science 276).

Indeed, from the tales of Sherlock Holmes to the scenes of CSI, forensic awareness is omnipresent in the popular media – and is something the public often raise in relation to crimes committed. But how much do solicitors or counsel really know about forensics – and could we provide a better level of service to clients – not to mention a better quality of examination of the case – if we knew more?

Most solicitors in criminal road traffic work will be familiar with the idea of challenging lab test results for drink or drugs, especially following recent high (and sometimes not-so-high) profile issues with accuracy of tests in various private forensic science services used in prosecutions. Similarly, almost all crime practitioners will have had to deal with the issue of DNA, fingerprints, blood, or other such forensic evidence in their cases to date – including the advent of Streamlined Forensic Reporting (SFR).

What do we know?

However, a study in 2016 by Davies and Piasecki found persistent problems with practitioners’ understanding of scientific evidence – which could be impacting on case outcomes. As they reflect: “Whilst great trust has been placed on the adversarial system as a trial safeguard, a significant body of case law would suggest that this trust has been misplaced… The 2005 report Forensic Science on Trial and the Law Commission’s 2011 Report clearly highlighted the ‘laissez faire’ attitude of the Bar and the judiciary to both the admissibility of expert evidence and the need for further training in respect of it...”

They go on to add: “It is suggested that training should start at the BPTC stage. The fact that there is no requirement for students to be trained either in basic scientific methodology or statistics or in relation to the advocacy skills required to examine an expert is a flaw in the current training system for barristers.” (Davies and Piasecki, “No more laissez faire? Expert evidence, rule changes and reliability: can more effective training for the bar and judiciary prevent miscarriages of justice?” (2016) 80(5) Journal of Criminal Law 327). The question could legitimately be raised of whether such a proposal, or at least the option to undertake such training, should extend to solicitors too – and this would be a fascinating facet in CPD.

On the scientific side, there has been much work done by Professor Niamh Nic Daéid et al. on the subject of practitioner and judicial forensic awareness, and how scientific and technical evidence is presented in court, beyond the parameters of the respective Procedure Rules. Indeed, as Professor Nic Daéid notes: “… there is an equal responsibility upon the lawyers not only to ensure clear communication but also to ensure that the appropriate forensic evidence is procured in the first place.” (Nic Daéid and Thorpe, “Teaching lawyers about forensic science” (1999) 100 Forensic Science International 149).

Touch transfer

The scientific field nevertheless remains a road less travelled for many legal practitioners. For example, did you know that use of a launderette can lead to DNA transfer and contamination, through shared, or subsequent but separate, use? (Noël et al., “DNA transfer during laundering may yield complete genetic profiles” (2015) 23 Forensic Science International: Genetics 240; Kamphausen et al., “Everything clean? Transfer of DNA traces between textiles in the washtub” (2015) 129 International Journal of Legal Medicine 709).

Or (despite its reduced frequency following covid-19), that social contact, especially handshakes, could potentially lead to the transfer of innocent DNA to a crime scene – and vice-versa? (Goray and van Oorschot, “DNA transfer during social interactions” (2013) Forensic Science International: Genetics Supplement Series 4 e101)

Sometimes, even the touching of communal objects, such as a door handle or a jug, can lead to DNA from an unknown third party individual to one’s hands – and it may not even be from the most recent prior user of the item (on door handles, see Fonneløp et al., “Secondary and subsequent DNA transfer during criminal investigation” (2015) 17 Forensic Science International: Genetics 155 – and on jugs, see Goray and van Oorschot, “The complexities of DNA transfer during a social setting” (2015) 17 Legal Medicine 82).

Fonneløp et al. found “‘touch-DNA’ can be transferred between multiple objects, and that disposable gloves can act as an efficient transfer vector. This study also found their results: “may indicate that transfer is not crucially dependent on handling-time as most of the transfer can happen with a few seconds of contact” – and that: “foreign DNA present on a person’s hand can be transferred onto a new object and follow the transfer chain to the third object.” (Fonneløp, Egerland and Gill, “Secondary and subsequent DNA transfer during criminal investigation” (2015) 17 Forensic Science International: Genetics 155).

The Tsekiri factors

In the domestic jurisdiction, the case of Tsekiri [2017] EWCA Crim 40 is highly relevant where the defence seek to raise the possibility of otherwise-explainable DNA transfer. In this case, the DNA of a suspect was found on a car door handle involved in an incident. The court held that, when there is a possibility of otherwise-explainable DNA transfer (i.e. that the Defendant’s DNA has somehow innocently come to be present at, or on, the scene), the Tsekiri factors should be considered.

The Tsekiri factors are:

  • Whether there is another explanation for the presence of the DNA evidence;
  • Whether the article is associated with the offence itself;
  • Whether the article is movable;
  • Any evidence of geographic association between the offence and the alleged offender;
  • In the case of a mixed [DNA] profile, whether the defendant is a match to the major contributor;
  • Whether primary or secondary transfer of the DNA is more likely.

Recent and reported cases

It is important to keep up with the news as to questions raised over other types of forensic testing, such as for drugs (BBC, “Randox forensics inquiry: Forty drug-driving offences quashed”; The Independent, “Faulty lab tests could put hundreds of drug driving convictions in jeopardy”:

Furthermore, reported cases such as Senior (Liam) [2018] EWCA Crim 837 and Wilson (Jamie Lee) [2019] EWCA Crim 2410, on drug driving allegations – and Richmond LBC v B [2010] EWHC 2903 (Fam), on alcohol abstinence hair testing in a family law context, show that forensics are not always absolutely flawless – and, with the best will in the world, sometimes things can go awry.

Streamlined Forensic Reporting (SFR)

In Hunt v CPS [2018] EWHC 3341 (Admin), it was held that SFR1s are only admissible if they are adduced as an agreed fact. In the absence of such agreement between the prosecution and the defence, they are wholly inadmissible in evidence. In the absence of an SFR2, it is foreseeable that the SFR1 may be the only ‘evidence’ of an individual testing positive for something. However, if the SFR1 is inadmissible by dint of not being an agreed fact, it follows that there is arguably insufficient evidence of presence in the Defendant’s system (or on their person, if there is an absence of other evidence).

In terms of SFRs, there is therefore no obligation for the defence to make use of an SFR1. The prosecution have to comply with Crim PR 19(3). The defence can request service of a SFR2 under Crim PR 19(4) – and the service of the full analytical data pack (Crim PR Part 19(3)(3)(d)).

In DPP v Stephens [2006] EWHC 1860 (Admin), it was confirmed that service of documents under s.16 of the Road Traffic Act 1988 (i.e. evidence of the proportion of alcohol or a drug in a specimen) is required to be personal, by registered post, or by recorded delivery, and to be at least seven days before the hearing. Any document under s.16 not so served, seven days before the hearing, is arguably inadmissible.

For SFR2s in s.16 cases: “A failure to serve not later than seven days before the hearing any of the documents specified in s.16(1)(a) cannot be waived: Tobi v Nicholas (1988) 86 Cr App R 323, DC; and the statute providing that the statement and/or certificate would be admissible ‘only if’ the prescribed steps had been taken, the fact that no objection to the evidence was taken in the Magistrates’ Court until the close of the evidence could not have been held to be a waiver of any right to object.” (Archbold Criminal Pleading Evidence and Practice 2021, [32-222]).

A failure to provide an SFR2 before the day of trial can thus result in an acquittal (Hunt v CPS [2018] EWHC 3341 (Admin).

Fireworks and firearms

Another area worth consideration is the effect of environmental and artefactual factors on forensic testing. In 2012, Grima et al. (2012) undertook a study: “…formulated to detect any possible particulate by random particle fallout onto substrates at firework displays and to assess the impact this may have on GSR evidence. Firework residue was collected at a display site, from amongst spectators as well as from the author’s hair 90 min after the display. SEM–EDX analysis has detected such particulate in all three scenarios, with the firework particle population at large providing a solid ground for discrimination from GSR. Wind dispersal was found to decrease the particle population and subsequently, the latter’s discriminatory power. Some particles, if treated individually were found to be indistinguishable from GSR… authorities such as police forces should be made more aware about the incidence of such particle transfer in firework related periods.” (Grima, Butler, Hanson, and Mohameden, “Firework displays as sources of particles similar to gunshot residue” (2012) 52 Science and Justice 49).

Their study further detailed: “The worrying factor was that a scientist might identify an environmental or occupational particle as GSR and hence, use it in court as proof that a suspect was in an area where a firearm was used. One source of such agglomerates is fireworks. Several studies have investigated possible similarities or differences between firework residue and GSR, with some exhibiting contradictory conclusions when compared to others.”

They continued: “The relative distance between spectators and fireworks is also important in particle distribution assessment… In the UK no such distances are in force, apart from a 25m safety distance… The presence of pyrotechnic particles that are indistinguishable from GSR also produce a risk to spectators who enjoy watching the colorful [sic] displays from the comfort of their home balconies or roofs [sic]... As a result, firework particles may find themselves inside the homes of these individuals.”

The study further commented about how emergency services personnel may be affected: “The most worrying form of contamination however, is in relation to the ambulance crews, fire brigade and police stationed at the site. Firework particulate may contaminate their clothing and vehicles. If these individuals are requested to respond to an incident after the display, especially a firearm-related one, possible contamination into crime scenes or onto victims or suspects may be inevitable. Contamination may be in the form of unknowingly planting a residue at a scene which may possibly indicate the use of a firearm or contaminate existing GSR, which may subsequently lead to its exclusion on the basis of pyrotechnic particle detection. Attending a scene the next day or days later may also have an impact on GSR evidence because the hygiene of these individuals may also play a role.”

Grima’s team went on to carry out a further study – Grima et al. (2014) – which, reflecting on their previous work, commented that: “…background particles can aid in the exclusion of firework particles which are indistinguishable from GSR.” (Grima, Hanson, and Tidy, “An assessment of firework particle persistence on the hands and related police force practices in relation to GSR evidence” (2014) 239 Forensic Science International 19).

The second study detailed matters further, noting: “…results showed that firework particles, both distinguishable and indistinguishable from GSR, can be expected to persist for at least one and a half hours on hair post display. During this period, fire fighting crews, ambulance crews and police officers stationed at a display site may be requested to enter a scene of crime post-display. This may result in unknowingly transferring firework particles into a scene of crime. Should any GSR evidence be contaminated with firework particles, it may be very complex for the expert to distinguish one population from another, especially since large numbers of firework exclusion markers are expected to be present. Given that firework particle populations are composed of particles that are both distinguishable and indistinguishable from GSR particles originating from the firearm may be classed as originating from fireworks, on the basis that several indistinguishable particles are surrounded by larger numbers of typical firework particles. The mixed population may therefore be excluded as pyrotechnic sourced, resulting in potentially strong evidence being lost.”

They added: “This investigation has proven that firework particles can remain on a person’s hands for long hours after sleeping, pending that they are not washed, an avenue which was not investigated within this study. Such particles were produced from both aerial and ground professional fireworks, as well as consumer grade fireworks. GSR evidence is already challenged by the existence of indistinguishable firework particles. The strength of background firework particle persistence continues to add strength to this challenge, particularly from the viewpoint of secondary contamination. The particles that exhibited persistence survived strong abrasive movements by the hands with the bedding. Police officers and members of both ambulance and firefighting crews may enter scenes of crime a few hours post-display. If they were stationed near or at a display site, there is a high possibility of them having pyrotechnic residues on their person, clothing and vehicles. Moving into a scene of crime with the same residues that exhibited strong persistence in the four scenarios of this project may contaminate an existing GSR residue at the scene or on a suspect, promoting possible difficulties in evidence interpretation… It is important for police forces to use showering facilities, as well as changing their clothing and vehicles if they are expected to continue working throughout the night. These measures will prove effective in minimising particle transfer.”

On the persistence of particles, the study noted: “The results obtained show a clear and significant persistence of firework particles because even though the subject was sleeping in bed for approximately 8h on each occasion, thus suggesting that particles would have been lost by contact with bed sheets, hundreds of particles still persisted on the hands…”

Reflecting on the significance of this, they add: “The persistence shown suggests that such particles may almost definitely be present on support crews who were stationed at display sites throughout the night. It is also expected that more particles would be present on the hands in the hours closer to the display. The fact that such firework particles have persisted presents a major factor to consider in relation to GSR evidence. If such particles are transferred by display site support personnel into scenes of crime related to firearm use, a GSR particle population has the danger of being contaminated with these pyrotechnic particles. One may argue that crime scene anti-contamination protocols are rigorous. However, reference is being made to first respondents and police officers and other personnel coming into contact with items and suspects. The contamination argument can also be applied to a suspect who fired a firearm, having also attended a firework display or been in the vicinity of any type of pyrotechnic activity. It may prove challenging for a GSR practitioner to confidently identify the GSR particle population from the pyrotechnic related one, present in the same sample. Furthermore it would get much harder when the number of GSR particles is much smaller that the clearly firework derived particle population…”

In terms of the practical implications for policing, the team add: “Considering that the detected persisted particles were recovered after the subject woke up at 8am the following morning, further transfer of these residues may occur. Within the policing sector, such transfer may still occur onto suspects and other officers as the day progresses. Police vehicles may also be subject to firework particle transfer. These may be used throughout the night and the next day for the transportation of suspects. Possibilities of secondary and tertiary contamination in such scenarios cannot be excluded even if only remotely possible.”

In terms of awareness of this issue, the study noted: “Even though GSR experts are in general aware of potential mistaken firework particle identity, the survey shows that this awareness has not made it into the majority of police forces… These findings show that procedures are lacking… While 28.6 per cent of police forces participated, the results are still extremely significant, as all participant police forces that send police officers to firework displays lack a preventive post-display particle transfer protocol.”

They also note: “For officers who remain on shift, other types of factors may play a role in particle persistence. However, if particles were still present after the four displays following a night of sleep with continuous hand contact and abrasive movements with bed sheets, it is expected that if the hands are not washed, police officers may still have particles on their person in the immediate hours post display. Supporting this argument is Grima et al.’s study that showed how particles were still present on the subject’s hair 90 minutes after the display. If personal protective equipment (PPE) is worn at a scene of crime, not washing the hands means that particles may be transferred to the PPE itself...”

They propose: “To prevent particle transfer it would be ideal for police forces to send different teams of officers to support firework displays. However, this would prove to be a costly measure for any force. Hence, it is being proposed that when police forces support firework displays, all officers have to wash their hands and hair, and use a different clean set of clothes when continuing work post display. Different vehicles should also be used for firework display attendance, especially if the same vehicles are used to transport suspects… Suspects transported post-display may have their hands

contaminated with these particles. This may lead to GSR on their hands being mixed with firework particles, possibly jeopardising potentially useful evidence. This may happen even if the suspect’s hands are covered with protective bags since firework particles transferred from the seats to the bags may get transferred to the hands upon removal. Moreover, any officer whose hands are contaminated with firework residues may unknowingly transfer them to the suspect.”

Grima’s team conclude: “Even if no forensic investigation has publically or academically disseminated the existence of firework contamination in casework, one cannot exclude that this

might have occurred following incorrect exclusions or from this happening in the future. Since the concept has been preliminarily introduced in a previous study and has been investigated in greater detail within this current project, extensive information required to identify such contamination was never at hand for GSR experts, solicitors and prosecutors.”


It is clear that the area of forensic education for lawyers has not, to date, been a priority. As Piasecki and Davies opine: “Too often, issues central to the assessment of scientific validity and reliability (and therefore probative value) have been circumvented by recourse to experience, formal qualifications, previous appearances in legal proceedings, previous involvement in investigations and convictions, the practice or jurisprudence in other jurisdictions, and the institutional practices and policies of police forces and forensic science institutions. These substituted factors may not however, provide actual evidence for the validity and reliability of techniques and derivative opinions, for, they do not provide independent evidence, or an actual guarantee, that a technique or method has probative value.” It remains to be seen, however, what, if anything, will realistically change in this respect.

Overall, this overview concludes by agreeing with Professor Nic Daéid that: “It is necessary to develop the evaluation skills of the legal profession in relation to forensic sciences in order for effective and accurate use of science based evidence within the legal process. It is our opinion that forensic science is not merely a matter of following a technical protocol to achieve a result but incorporates a thought process involving an appreciation of the relevant of scientific findings, their limitations and significance… in order to efficiently and effectively use evidence correctly the lawyer needs to have a basic grasp of relevant scientific principles. In the legal context, Locard’s Exchange Principle springs to mind…”

Chaynee Hodgetts FRSA is our Features and Opinion Editor, Honorary Lecturer in Law, and Mature Pupil Barrister with Nexus, the Chambers of Michael Mansfield QC:;;