Acne is a chronic inflammatory disease of the sebaceous hair follicles. Acne vulgaris is caused by an excessive amount of oil production, blockage of the openings to the sebum producing glands (resulting in blackheads) and a bacterium, Propionibacterium acnes (P. acnes) which causes inflammation in the blocked glands. These three factors produce the red and yellow spots. Some Lasers work by killing the bacteria, and others help to reduce the production of sebum from the sebaceous glands in the skin. Intense Pulsed Light (IPL) appears to have an antibacterial effect. Light Emitting Diodes (LEDs) can produce blue light that helps to destroy the P. acnes bacteria. Photodynamic Therapy (PDT) uses a cream called aminolevulinic acid (ALA) which is applied to the skin prior to the application of a high-intensity red LED light or laser/IPL. PDT seems to improve acne by decreasing the bacteria on the skin and by shrinking the oil-producing sebaceous glands. Prices vary depending on the light source. Costs can be £75 - £300 per session.