Effect of wind on region of origin calculations of impact bloodstain patterns: implications for bloodstain pattern analysis

All forensic investigators understand the relative importance of blood within a crime scene. Blood is one of the most common and important types of physical evidence present at a crime scene and an investigator can derive valuable evidence and information. The reconstruction of a bloodshed event is often used to determine physical events that have occurred. One such reconstruction tool involves the determination of the Area of Origin (AOO). The AOO is the three dimensional location from which the blood that produced a bloodstain originated. Strings or virtual strings are projected from the leading edge of the stain at the impact angle, while maintaining the gamma angle (which is along the axis through the ellipse). Extensive research has been conducted into the methodology employed to validate the accuracy of AOO determinations. However, no research has studied the effect of wind on AOO determinations that are routinely conducted in many crime scene reconstructions.

The present study aims to identify if the presence and intensity of wind affects the flight path of airborne blood droplets, and therefore, the calculated AOO. A total of 70 impact bloodstain patterns were created in association with seven wind speeds ranging from 0 to 17 kph, a fan placed 90 degrees to the receiving surface was used to generate the wind source. The Tangent method was then used for AOO determinations, with Microsoft Office Excel 2003 Auto Shapes being utilised for stain measurement and angle of impact calculations. The 70 impact bloodstain patterns were divided into seven groups of 10 according to the wind speed. The known X, Y and Z coordinates were compared against the calculated X, Y and Z coordinates and the direction difference determined (positive or negative).

The results of this project demonstrated that the presence and intensity of wind affects the AOO determination for impact bloodstain patterns, in 4/60 patterns this resulted in the determination of a 2nd AOO, thus indicating a second impact where there was actually none. This may have been a coincidence as there appears no reason to expect blood drops to divert in such a way to give a second convergence area. why the blood drops The research also identified that the distribution of individual bloodstains within an impact bloodstain pattern were affected by wind, with the directional features of many individual bloodstains indicating that these bloodstains were not resultant of the known impact that occurred. Forensically the latter is extremely important, because if the BPA analyst is not cognisant of the effect of wind, these individual bloodstains would be interpreted as originating from a different incident than that of the actual impact.