Security Scanners

1. Introduction

Increased concerns for terrorist attacks has led to the development of new passenger screening methods.

Passengers are routinely screened at airports using either hand searches or metal detectors. Sometimes, techniques that detect traces of explosives or sniffer dogs are also used. Because of increased concern over terrorist attacks, some countries have introduced more efficient screening devices such as scanners.

There are 4 different types of scanners currently available in the market:

Millimeter-wave scanners, that don’t use X-rays:

1. Passive scanners detect the very low levels of natural radiation emanating from the body surface, the scanners themselves do not emit any kind of radiation.
2. Active scanners emit radio waves that are reflected back by the body surface. These radio waves are not ionizing.

X-ray scanners:

1. Backscatter scanners emit low energy X-rays that are reflected back by the body surface.
2. Transmission scanners send higher energy X-rays through the body in the same way as a traditional medical X-ray machine and can reveal objects inside the body.

Security scanners have been used at airports in Russia for a few years and are being introduced or considered worldwide, especially in the USA. In Europe, a scanner is being trialled in a UK airport but X-ray based scanners are banned in several member states that don’t allow the use of ionising radiation for non-medical purposes.

X-rays are one type of ionizing radiation, which also include the radiation from radioactive materials. It is called ionizing because the particles that compose it have enough energy to knock out an electron from a molecule, turning it into an ion by giving it an electric charge. This ion can then react with other molecules, which can cause damage to the components of a cell, and alter its functioning. When these reactions affect the genetic material of a cell, then there is the possibility of causing cancer, and this is the main point of concern for low-levels of radiation.

Radiation is present everywhere and as an example, all Europeans receive a dose of approximately 1 mSv a year from naturally-occurring radiation in the environment. People are also exposed to radiation from space, particularly at high altitudes and when flying. Breathing in indoor radon also contributes 0.1 to 10 mSv of radiation every year.

The main man-made sources of radiation are scans and X-rays for medical diagnosis, and radiotherapy. The doses received from medical diagnosis vary and can be high for some individuals but in general they affect very few people so it is not relevant to calculate population averages.

The harmful effects of exposure to high doses of ionising radiation, above hundreds of mSv, are well known but X-ray security scanners result in very small doses, of the order of a thousandth of a mSv. The cancer risks for such small exposures are not measurable but can be estimated by extrapolating the data from higher doses and assuming that the effects are directly proportional to the exposure and that there is no safe limit of radiation. More...