Contact dermatitis occurs when your skin reacts to a particular substance. This can be either:

• an irritant – a substance that directly damages the outer layer of skin
• an allergen – a substance that causes your immune system to respond in a way that affects the skin

Irritant contact dermatitis

Irritant contact dermatitis may be caused by frequent exposure to a weak irritant, such as soap or detergent. It may also develop if you've been in contact with a stronger irritant for a short while.

You're at an increased risk of irritant contact dermatitis if you also have atopic eczema, which is the most common form of eczema.  Common irritants include:

• soaps and detergents
• antiseptics and antibacterials
• perfumes and preservatives in toiletries or cosmetics
• solvents
• oils used in machines 
• disinfectants
• acids and alkalis
• cement
• powders, dust and soil
• water – especially hard, chalky water or heavily chlorinated water
• many plants – such as Ranunculus, spurge, Boraginaceae and mustards

If you already have irritant contact dermatitis symptoms, they can be made worse by heat, cold, friction (rubbing against the irritant) and low humidity (dry air).

Exposure at work 

You may be more at risk of irritant contact dermatitis if you work with irritants as part of your job, or if your job involves a lot of wet work. If you develop the condition because of a substance you work with, it may be referred to as occupational irritant dermatitis. This type of dermatitis is more common in certain occupations, including:

• agricultural workers
• beauticians and hairdressers
• chemical workers
• cleaners
• construction workers
• cooks and caterers
• metal and electronics workers
• health and social care workers
• machine operators
• mechanics and vehicle assemblers

Allergic contact dermatitis

The first time you come into contact with an allergen, your body becomes sensitised to it, but does not react to it. It's only when you're exposed to the substance again that your immune system reacts and causes the skin to become red and itchy.

Allergens that commonly cause allergic contact dermatitis include:

• cosmetic ingredients – such as preservatives, fragrances, hair dye and nail varnish hardeners
• metals – such as nickel or cobalt in jewellery
• some topical medicines (medicines applied directly to the skin) – including topical corticosteroids, in rare cases
• rubber – including latex, a type of naturally occurring rubber 
• textiles – particularly the dyes and resins that are contained in them
• strong glues – such as epoxy resin adhesives
• some plants – such as chrysanthemums, sunflowers, daffodils, tulips and primula