Therapy uses essential oils and water-based colloids extracted from plant materials to promote physical, emotional, and spiritual health and balance. Single or combined extracts may be diffused into inhaled air: used in massage oil, or added to bathwater. Inhaled molecules of these extracts stimulate the olfactory nerve, sending messages to the brain's limbic system (the seat of memory, learning and emotion) that are said to trigger physiological responses (e.g. eucalyptus relieves congestion: lavender promotes relaxation). Mainstream medical practitioners question the claim of independent physiological effects; they consider many of the benefits are more likely due to the conditioned responses that the odours can reinforce or help create. The oils and solutions used have been shown to have certain effects but are not standardized. The few risks involved include allergic reactions.