Polypropylene is generally considered a safe plastic for human contact under normal conditions. It is used in various applications, including food containers and textiles. When it comes to inhalation, the normal use of products made from polypropylene is not associated with toxicity. However, the burning of polypropylene can release toxic fumes, including formaldehyde, acrolein, and other volatile organic compounds that can irritate the respiratory tract and pose health risks with prolonged exposure. Therefore, while polypropylene itself is not toxic to breathe in its solid state and during regular use, inhaling fumes from burning or melting polypropylene should be avoided.

Titanium dioxide and zinc oxide are considered safe and effective as sunscreen ingredients. These minerals work by providing a physical barrier that reflect or scatter UV radiation. They are less likely to cause skin irritation and allergies, thus recommended for sensitive skin. However, some concerns were raised about their nano forms, which could potentially penetrate skin. Overall, the evidence suggests that they remain on the surface of the skin and do not enter the body, ensuring their safety. For cosmetics involving inhalation exposure, such as powders, the safety of nanoparticle use is still under scrutiny.

PVC pipes, known for their durability, affordability, and versatility, come in various types catering to diverse applications. The main types include:
1. **Unplasticized PVC (uPVC)** - Rigid and most commonly used for water supply lines and sewerage systems, it's known for its resistance to chemicals and water corrosion.
2. **Chlorinated PVC (CPVC)** - With added chlorine, CPVC offers higher temperature resistance, making it suitable for hot and cold water supply.
3. **Molecularly Oriented PVC (PVC-O)** - This is uPVC subjected to a special orientation process, enhancing its strength and making it ideal for high-pressure applications.
4. **Flexible PVC** - Incorporating plasticizers, this type is more flexible and is often used for wiring or as tubing in medical devices.
Each type's suitability varies depending on the application's specific requirements, such as temperature exposure, pressure needs, and chemical resistance. Choosing the right PVC type ensures efficiency, safety, and longevity in its application.