evidence: Vaccine ingredients can look unfamiliar. However, it is important to remember that many of the substances used in vaccines are found naturally in the body. For example, many vaccines contain salts based on sodium and potassium (see the section on 'Acidity regulators'), which are essential for life. People may think of formaldehyde as a man-made chemical, but in small quantities, it is also found naturally in the bloodstream. [...] If you look up some vaccine ingredients on the internet you may read that they could be harmful, but most of them are present in vaccines in amounts that are completely normal for our bodies. Even common salt (sodium chloride), which is essential for the normal functioning of the body, is harmful in large quantities. [...] Formaldehyde is an organic compound found naturally in many living things. It is used in the production of some vaccines to inactivate toxins from bacteria and viruses (for example, poliovirus, Hepatitis B antigen, and diphtheria and tetanus toxins). It is possible that tiny traces may remain in the 6-in-1 vaccine (Infanrix Hexa), Hepatitis B vaccine (HBVaxPro), one of the Pre-school Booster vaccines (Repevax), and the Teenage Booster vaccine (Revaxis). However, formaldehyde breaks down quickly in water (and most of the vaccine is water).