Food hazards can be classified into three categories: physical, chemical and biological. Physical hazards (e.g. stones in rice or beans; bone pieces in meat) are most likely to be understood by people. Far more complex and less understood is the nature of the impact of chemical and biological hazards oh human health because of the complexities of the interaction between the hazard and human biochemistry and the absence of empirical data to confirm the theories. Human responses to disease or agents that cause adverse reactions are dependent on a number of variables, many of which are interlinked. Also, one person may experience severe health effects, another mild effects and a third no effect at all.