Which of the following is NOT a hazard associated with pre-blended oils?

Pre-blended oils are typically formulated with specific safety considerations in mind, including the balance of essential oils and carrier oils to minimize hazards. When considering the option that is not a hazard of pre-blended oils, infection is the one that stands out.

Infection is usually associated with unsanitary practices or the presence of pathogenic microorganisms, rather than a direct result of the oils themselves. Pre-blended oils, especially those that are commercially prepared, are often processed and packaged under controlled conditions to minimize any risk of contamination. This is unlike toxicity, irritation, and photo-sensitivity which can arise from the essential oils and their concentrations within the blend.

Toxicity can be a concern if the oils contain components that may be harmful in certain situations or quantities. Irritation can occur if sensitive skin comes into contact with certain essential oils, particularly non-diluted forms. Photo-sensitivity refers to the reaction that some essential oils can cause when the skin is exposed to sunlight after application, often exacerbated by specific components in the oils. Thus, the potential for infection does not inherently relate to the characteristics of pre-blended oils themselves, making it the correct choice in this context.