Acetaldehyde is a substance that is produced in the human body during metabolic processes, for example when the body breaks down alcohol. This substance often occurs in nature as a chemical by-product in plants and in many organisms. It is also a natural ingredient in many foods, such as fruit, coffee and bread. The taste of acetaldehyde is described as fresh with a fruity but sometimes musty odour.

At room temperature, acrolein (also called propenal or acrylaldehyde) is a colourless or yellow liquid. Acrolein is produced when organic materials such as tobacco are burned incompletely. Under these circumstances acrolein is a gas and can be inhaled. It is a very reactive substance with an unpleasant, penetrating, pungent odour.

Benzene is a colourless liquid with a sweet odour. Benzene evaporates quickly, dissolves poorly in water, and is quite flammable. Benzene is present in the environment: in water, air and the soil. The substance was discovered in the 19th century and is extracted from coal tar. It is currently made primarily from petroleum. It is one of the most commonly produced chemical substances and is used as a raw material for making other chemicals, such as ethylbenzene, from which styrene and eventually plastics are made. Benzene can also be formed by natural processes, such as in volcanoes and forest fires.

BaP belongs to the group of compounds known as polycyclic aromatic hydrocarbons (PAHs). PAHs are produced when organic material is burned incompletely or heated. Exhaust fumes from (diesel) vehicles and smoke from wood stoves contain PAHs, which are usually attached to dust particles. PAHs occur in the air, in water, in the soil and in sediment, and in some cases in food products as well.

1,3-butadiene, or simply butadiene, is a colourless, flammable gas with an odour resembling that of petrol. Butadiene is used as a raw material for the production of synthetic rubber in the car tyre industry and for making such plastics as nylon. Butadiene is released into the atmosphere through industrial processes, during forest fires, and when wood is burned. Once butadiene is released into the atmosphere, it quickly decomposes through exposure to light.

Carbon monoxide (CO) is a colourless and odourless poisonous gas. Besides smoke from tobacco, known sources of exposure to CO include exhaust fumes from cars, gas stoves, wood stoves and heaters. CO is also formed by natural processes and is released into the environment or into the (human) body.

Formaldehyde is an organic compound formed through natural processes in the environment and in the (human) body, where it is released through metabolic processes. Formaldehyde is also produced and used in various branches of the chemical and pharmaceutical industry. Formaldehyde is found in products including particle board, MDF (Medium-Density Fibreboard), textiles and cosmetics.

NNN and NNK are among the most important of the so-called tobacco-specific nitrosamines. These compounds are formed in tobacco leaves from nitrite and amines (proteins). Thus, NNN and NNK are not added to tobacco, but are produced through a natural process.

Execution framework The execution framework describes how the colorectal cancer screening programme and the subsequent care are carried out so that it runs effectively and within legal, policy-related frameworks. The document specifies the quality frameworks that are necessary to provide citizens with a uniform, reliable and high-quality population screening and related care.  The implementation framework is aimed at all professionals who are involved in the colorectal cancer screening programme and subsequent diagnostics and in the quality assurance, monitoring and evaluation of the screening programme. Factsheet In 2014, a colorectal cancer screening programme was launched in the Netherlands. We developed the following factsheets to inform international professionals about this screening programme: Lessons learned This factsheet describes the lessons learned during the introduction of this nationwide programme. The factsheet illustrates the most important measures that were taken in the Netherlands and provides a range of practical tips for policymakers in other countries. Overview of programme structure This factsheet describes the five stages of the screening programme and gives a detailed overview of the process, including the parties involved, the materials, working documents and protocols.  Overview of quality assurance This factsheet describes all the quality assurance measures within the Dutch CRC screening programme.