When purchasing wholesale pig hair, the most critical factors are length (tops), stiffness, and color. Natural bristles are categorized by their 'tops' percentage, which indicates the proportion of hairs reaching the full length; 90% tops represent premium quality. You must also specify the stiffness (soft, medium, or stiff) based on the application, such as industrial brushes or high-end hairbrushes. Additionally, ensure the hair has undergone proper sterilization and scouring to remove natural oils and odors, meeting ISO 9001 production standards.