## Common ground
All options are types of air filters used to remove contaminants from air in various applications.
## Options at a glance
**5603.94 – Nonwoven chemical-bonded**
- **Defining trait:** Made from nonwoven fibers bonded by chemicals, not woven or stitched.
- **Typical examples:** Disposable HVAC filters, industrial dust filters.
- **Best fit when:** The filter uses synthetic nonwoven material with chemical bonding (e.g., melt-blown polypropylene).
**5911.90 – Textile air filters**
- **Defining trait:** Constructed from woven or knitted textile fibers (e.g., polyester, cotton).
- **Typical examples:** Reusable fabric filters, HVAC filters with fabric media.
- **Best fit when:** The filter is made of woven or knitted fabric, often washable and reusable.
**8421.31 – Automotive engine air filters**
- **Defining trait:** Specifically designed for internal combustion engines in vehicles (e.g., intake air filters).
- **Typical examples:** Car engine air filters, oil-air filter combos for cars.
- **Best fit when:** The product is used in automobiles or motorcycles to protect engines.
**8421.39 – Air filtering machinery/apparatus**
- **Defining trait:** Part of larger systems that clean or purify air (e.g., HEPA units, industrial air purifiers).
- **Typical examples:** Replacement filters for air purifiers, industrial air cleaning systems.
- **Best fit when:** The filter is a component in a machine designed to purify air (not just trap particles).
## Key differences
| Feature | 5603.94 | 5911.90 | 8421.31 | 8421.39 |
|-----------------------|------------------|-----------------|---------------------|----------------------|
| Material type | Nonwoven, chem-bonded | Woven/knitted textile | Automotive-specific | Part of air purification system |
| Primary use | General filtration | Reusable filters | Engine intake | Air purification systems |
| System integration | Standalone | Standalone | Vehicle-specific | Integrated into machinery |
## How to choose
1. Is the filter made from nonwoven fibers bonded by chemicals? → 5603.94
2. Is it made from woven or knitted fabric (e.g., polyester)? → 5911.90
3. Is it used in a car, motorcycle, or engine intake system? → 8421.31
4. Is it part of an air purifier or industrial air-cleaning machine? → 8421.39
## Overlaps & pitfalls
- A filter may be both textile (5911.90) and used in a purifier (8421.39), but the **main function** determines the code.
- Automotive filters (8421.31) can be made from nonwoven or textile materials, so material alone isn’t enough—context matters.
- If unsure, prioritize **application**: engine use → 8421.31; air purification system → 8421.39