Pollen and Pollen Rain

Pollen is an important form of trace evidence , which can help link a suspect to a crime scene. This branch of forensic science has developed alongside advances in microscopy. Experts in forensic pollen analysis are called palynologists. They can determine whether the pollen species and patterns found on a suspect are characteristic of a particular area. It is not just the identity of the pollen that is important, but also the way in which it is dispersed, known as pollen rain. Each area has its own type of pollen rain that depends upon its native flora.

Pollen is the male sex cells of flowering or cone-bearing plants. It is microscopic and found on nearly every surface and object, so suspects will be carrying it, unknowingly, on their clothes, hair, and body. Pollen is also found on victims and on significant items such as ransom letters and money involved in crimes like bank robberies or drug dealing. The investigator has to know where to look to collect pollen samples; good places include any samples of soil, dust, mud or dirt, on clothing or perhaps in the suspect's vehicle.

Each plant spreads its pollen in a different way and a different plant ecology is found in each region. Wind-pollinated plants produce a lot of pollen, while self-pollinated and insect-pollinated plants produce much less. These properties lead to the characteristic pollen rain patterns of different regions.

Pollen rains down continually and can contaminate the sample containing the pollen of interest. It has been found useful to brush the desired sample with a clean, dry, cosmetic brush to get rid of this contaminating pollen that has nothing to do with the crime event. Then a sample of the pollen-containing material is scraped or brushed into a clean container. If the sample is dust, then a lift onto adhesive tape might be made. Hair is a very good source of pollen. Every time wind blows through someone's hair, pollen clings to it. The pollen sample can be washed off a hair sample with detergent. Pollen can also be found on many other surfaces which may be relevant, such as blankets, carpets, and packaging, including envelopes, and can be brushed or scraped off. The usual precautions in handling trace evidence apply—the chain of custody of the evidence—must remain intact if the evidence is to be admissible in court. Great care must be taken to prevent contamination; this is particularly important with pollen because the investigators will also have pollen on their own clothes and hair.

Examination of pollen from a victim sometimes provides evidence not otherwise readily available. Since pollen settles on food, analysis of pollen found in stomach content can give a clue as to where the individual was just prior to their death. Since pollen takes a long time to decay, samples taken from decomposed and even skeletal remains can still be informative.

It is important that the investigator collects plenty of control pollen samples from the scene. This will provide a baseline of the pollen type and pollen rain expected for that area. The forensic samples are compared to these and so help determine their relevance. If a body has been moved, for instance, the pollen will differ from that which is characteristic of the place where it is found.

Once back in the lab, the pollen has to be extracted from the evidence for microscopic examination. There are standard ways for doing this, but they are usually destructive of the evidence. If that particular piece of evidence, such as hair, has to be subjected to other analysis, then the pollen analysis must be done last. Microscopic evidence can identify a pollen grain by comparison with standard samples held in a database. The pollen rain pattern can also be identified by looking at the different pollens present and their density. Low density of grains suggests self-pollinating species; high density suggests wind-pollinated species.

In one early case, which was solved with the help of forensic palynology , a man had disappeared near the Danube River in Vienna in 1959. There was a suspect with a motive, but no body had been found and the suspect denied any crime. However, the investigators found mud on the suspect's shoes, revealing spruce, willow, and alder pollen, as well as a fossil hickory pollen grain that had survived for millions of years. Only one small area in the Danube valley had this particular pollen mix. When confronted with this fact, the suspect broke down, confessed, and led police to the body that was, indeed, buried in this area.

see also Botany.