Here's why:

* SPF Rating: Zinc oxide in ointments typically does not have a high enough concentration to provide adequate sun protection. Sunscreens must meet specific criteria and have an SPF rating to be effective.

* Formulated for Skin Protection: While zinc oxide is an active ingredient in some sunscreens, it's often combined with other ingredients and formulated differently to provide the necessary UV protection.

* Application and Coverage: Ordinary zinc oxide ointment is usually thick and difficult to spread evenly, which makes it challenging to achieve the required coverage for sun protection.

Important Note: Using ordinary zinc oxide ointment as a sunscreen can lead to insufficient protection from harmful UV rays, increasing the risk of sunburn and long-term skin damage.

For reliable sun protection, always use a sunscreen with an SPF of at least 30 and broad-spectrum protection against both UVA and UVB rays.