Roundup Weed Killer contains a chemical known as Glyphosate.
When you get Roundup on your skin that Glyphosate can be absorbed into your skin and lead to health problems as well as causing skin and chemical burns if not washed off right away.
If you do happen to get some roundup on your skin you need to wash the skin using some soap right away or as soon as possible after coming into contact with the roundup.
Roundup has been shown to cause cancer in some people so it's recommended to wear some long rubber gloves as well as wear a mask when spraying roundup.
Breathing Roundup can also be harmful and cause cancer such as Melanoma or other forms of cancer.
So if you do need to use Roundup make sure to protect your lungs and skin from the roundup to prevent health problems and chemical burns.