Humans are exposed to many chemicals on daily basis but mostly toxicity obtained from single chemical exposure. Many factors are to be considered such as duration, route, frequency, timing of exposure, concentration while determining toxicity. To better understand the toxicity of chemical mixtures, a proper model system is essential. Shortcoming model systems are facing difficulties to find out appropriate balance of rigor and reproducibility in mixture toxicity studies. Certain questions will arise while comparing single to mixture toxicity. The most sensitive cases where toxicity leads to occur at lower doses is called as critical effect. They are based on the synergism, additivity, antagonism and potentiation. For toxicity evaluation to be more accurate, dose or concentration relevance should be considered and be below the threshold. Previously the work on chemical substances in done by the technology and methodology of the time but the recent studies are based on the progress of chemical substances in the mixture toxicology studies. Epidemiological risk assessment test are based on the chemical analytical tests. The evaluation of chemical substance is done by the quantitative or qualitative results. Amount of dose that is given to particular individual should be known for the study during exposure. This review consists of methods to determine the toxicity of using certain chemical substances and their epidemiological studies and functioning of chemical substances on human organs. Keywords: Toxicity, Chemicals, Dose, Synergism, Antagonism.