VOCs (volatile organic compounds) are the fumes that you smell when a paint job is on in your house. It can cause headaches and allergic reactions. Oil-based paints have a higher VOC content. If you must use oil-based pint, choose one with a VOC content of less than 380gm per litter. Or simply use an oil-based primer with a water-based paint to get the benefits of both.
When in doubt, ask!
Seek out a knowledgeable paint representative at your local hardware store who can clear your doubts and also offer advice about application. These numbers are usually on the label or on the official product literature. If the data is not available, consider choosing another brand.