Secondary milia look similar to primary milia but it develops because of clogged sweat ducts in the skin. This usually happens after there has been some kind of trauma to the skin, specifically any time your skin blisters such as with a burn, poison ivy rash, or a harsh session of dermabrasion.
Sunburns and sun exposure are also common causes of milia. For example, sun exposure damages the surface layers of the skin, and that damage causes the skin to thicken. When dead skin cells try to make their way to the surface of the skin to flake off and can't – there's that thick skin in the way -- they begin to form small white cysts of keratin as the skin cells build up. These cysts are milia.
Some chronic skin disorders are known to be associated with the appearance of milia. Tattoos, use of strong topical corticosteroids, and allergic rashes such as contact dermatitis have also all been known to trigger the development of secondary milia.