Over the last 10 years, blood pattern analysis (BPA)’s foundation in physics and other fields of science has been called into question . Current BPA reconstruction methods utilize straight trajectories in crime scene reconstruction, neglecting to account for the effects of gravity and drag which results in the actual trajectory being curved rather than straight. Further, BPA faces several environmental obstacles to accurate reconstruction, such as humidity levels, the properties of the air at the time, and similar blood patterns resulting from very different sequences of events. BPA also fails to account for certain factors, such as the effect that cooling of blood drops mid-flight may have on BPA. There are also concerns that the International Association for Identification (IAI) has no strict educational requirements for BPA certification. This may lead to ambiguity in blood pattern analysis cases, where the blood pattern analysis is brought up as part of a forensic or criminal investigation.
In 2009, a study by the National Academy of Science found that “the opinions of bloodstain pattern analysts are more subjective than scientific .” This claim matches with BPA’s failure to fully consider scientific and physics principles into its analysis.
One of the most commonly used BPA techniques is “stringing”. However, the use of stringing creates a systematic error by using straight lines to predict the vertical region of origin, overestimating the “height of the point of origin” with a significant margin of error (greater than 50% on average). The stringing “straight line” method creates this error because the actual path of flight is curved due to the effect of gravity, an effect amplified by air drag. Further, initial drop velocity and direction cannot be established from available bloodstains, making a prediction of drop trajectory even more difficult.
BPA analysis is also made less certain by environmental factors at the scene. Humidity levels can have a considerable effect on the drying time of blood droplets. At 60% humidity or greater, drying time is altered considerably and was found to slow “as humidity increased from 12-66.5%”. As such, as the age of the blood droplet increases, even scientific methods such as spectroscopy become increasingly unreliable in predicting the age of droplets. Air motion is another environmental factor that decreases the reliability of BPA. The air the blood moves through is not always still and may sometimes affect the path of the blood.
Misidentification and Physiological Factors
Another issue with BPA is the risk of misidentification. Blood that has been expirated (such as when there is bleeding in the mouth or throat) forms splatter patterns producing a fine mist, which can be misidentified as high-velocity impact spatter. Viscosity and surface tension both factor into how blood acts in flight as well as how a drop spreads or splashes upon impact. However, BPA fails to account for factors affecting viscosity. Changes in blood temperature were found to have an effect on both viscosity (60%) and surface tension (27%) when changed from 37°C (human body temperature) to 20°C (an average indoor air temperature). Furthermore, the viscosity of human blood can vary between individuals due to factors such as red blood cell count, possibly affecting how a blood drop may act during a splashing event.
There is a further concern with the requirements for BPA analysis certification. The IAI only requires a certain number of hours of training to be licensed to do BPA analysis. There is no required testing of forensic investigation skills relating to BPA. This lack of educational requirement seems at odds with the “importance of rigorous and objective hypothesis testing and the complex nature of fluid dynamics.”
While BPA analysis may aid in crime scene reconstruction, the practice has been criticized for its disconnect with the science and physics behind blood patterns. Furthermore, the loose licensing requirements for BPA analysis certification seem out of place considering the complex nature of fluid dynamics in flight as well as the importance of extensive and rigorous testing for criminal evidence. Blood pattern analysis cases, where blood pattern analysis plays a key role as forensic evidence, may warrant greater scrutiny.
This piece was written originally by Dilan Brar. Currently, Dilan Brar is a second-year law student at the University of Toronto.
- 1 Committee on Identifying the Needs of the Forensic Science Community, Strengthening Forensic Science in the United States. (Washington, DC: The National Academies Press, 2009).
- 2 See Comiskey et al, “Prediction of blood back spatter from a gunshot in bloodstain pattern analysis” (2016) 1:4 Physical Review Fluids 043201 at para 3-4, online.
- 3 Dana Dryzal, “Bloodstain Pattern Analysis: Applications and Challenges” (2018) 2:2 D.U.Quark 22-29, online.
- 4 Supra note 2.
- 5 Ibid at 043201-3.
- 6 Supra note 1 at 178.
- 7 Ibid.
- 8 Deidre Sieden “Blood in flight” (13 January 2017), online Royal Canadian Mounted Police.
- 9 Supra note 2 at 043201-2
- 10 Ibid.
- 11 See Laux et al, “Ultrasonic monitoring of droplets’ evaporation: Application to human whole blood” (2016) 32 Ultrasonics Sonochemistry 132-136 at 134.
- 12 See Zeid et al “Influence of evaporation rate on cracks’ formation of a drying drop of whole blood” (2013) 432 Colloids and Surfaces A: Physicochemical and Engineerings Aspects 139-146.
- 13 Supra note 3.
- 14 Supra note 2
- 15 Supra note 3 at 25.
- 16 Supra note 2.
- 17 Ibid.
- 18 Alison Balance “The physics of blood spatter” (2 November 2017), online Radio New Zealand.
- 19 Supra note 1 at 178.
- 20 Ibid