Why they're unhealthy: Nitrates have been linked to infant methemoglobinemia, a deficiency of oxygen in the blood. Also, nitrate can interact with the amines in protein to form nitrosamines, which have been linked to cancer in lab animals.
What's being done: The USDA now requires bacon processors to add antioxidants, such as vitamins C and E, which have been proven to inhibit nitrosamine formation. But if you're still worried, you can limit your intake by sticking with nitrate-free bacon products, having your bacon with a glass of antioxidant-rich orange juice, and not overcooking your strips. Studies have shown that well-done bacon contains more nitrosamines.