Contamination of oils normally results as a consequence of the addition of chemicals to the plant or soil during the growing stage. These chemicals may be added for a number of reasons:

• Fertilizers added to enrich the soil.
• Herbicides added to the soil to prevent growth of weeds.
• Pesticides sprayed on the plant to deter insect pests.
• Plant hormones sprayed on plant to stimulate a particular growth stage
  e.g. to encourage all the plants to fruit at the same time to aid harvesting

Many of these chemicals can be absorbed into the plant and carry over into the essential oil during distillation. There is some evidence to suggest that problems such as allergic reactions are due to these chemicals and not the oil itself. However it is worth remembering that these chemicals are also present in foods and the level in oils is normally much lower.
Many aromatherapists look for terms such as 'naturally' or 'organically grown' when purchasing an oil. However there is no agreed definition of these terms and virtually any oil can claim to be organic or natural. There are organisations such as the Soil Association who set international standards for crop growth and it is worth looking for oil producers that meet such requirements.