Filters are made from a wide range of materials depending on what they need to remove and from what. Water filters typically use activated carbon, ceramic clay, or thin plastic membranes. Air filters rely on glass fibers or electrostatically charged polypropylene. Oil filters use cellulose (wood pulp) fibers, sometimes blended with synthetics. The material always comes down to one goal: creating a structure with pores small enough to trap unwanted particles while still letting the desired fluid pass through.
Water Filters: Carbon, Ceramic, and Membranes
The most common household water filters use activated carbon, a highly porous form of carbon that traps contaminants as water passes through it. Activated carbon can be made from coal, wood, or coconut shells. Coconut shell carbon is the most expensive but also the most effective, with a tighter pore structure that catches a wider range of chemicals including chlorine, pesticides, and organic compounds that affect taste and odor.
Ceramic water filters take a different approach. They’re made from natural clay mixed with materials like sawdust, limestone, or diatomaceous earth, then fired in a kiln at temperatures between 850 and 1,100°C. The firing process burns away the organic material, leaving behind a rigid structure full of tiny pores, typically between 0.2 and 1 micrometer across. Those pores are small enough to physically block bacteria, achieving removal rates above 99.99% in well-made filters. Many ceramic filters are also coated with silver nanoparticles, which kill bacteria that get trapped on the surface and prevent colonies from growing inside the filter over time.
Reverse osmosis systems, the type found under some kitchen sinks, use a thin plastic membrane made primarily of polyamide, a type of nylon, layered over a porous support. This membrane has pores so small they block dissolved salts, heavy metals, and most contaminants at the molecular level. The tradeoff is that water must be pushed through under pressure, and a significant portion goes down the drain as waste.
Air Filters: Glass Fibers and Charged Plastics
HEPA filters, the gold standard for air purification, are made from borosilicate glass fibers. These extremely fine glass strands, typically 3 to 20 micrometers in diameter, are formed into a dense mat of filter paper. The technology dates back to the 1950s, when glass fiber paper replaced asbestos as the filtering medium, and the basic approach hasn’t changed much since. The fibers are held together with chemical binders that help the filter maintain its structure under heat, moisture, and airflow. HEPA filters capture at least 99.97% of particles 0.3 micrometers and larger, which includes dust, pollen, mold spores, and most bacteria.
The flat panel filters in home HVAC systems use a coarser version of similar fibrous materials, often fiberglass or polyester, arranged in a less dense mat. They catch larger particles like dust and pet hair but let smaller ones pass through.
N95 and Surgical Masks
The critical layer in an N95 respirator is melt-blown polypropylene, a plastic formed into extremely fine fibers between 0.5 and 4 micrometers in diameter. During manufacturing, electrical charges are injected into the material, creating a semi-permanent electric field across the fibers. This electrostatic charge is what makes N95s so effective: particles are attracted to the fibers the way a balloon sticks to a wall after being rubbed on your hair, catching particles far smaller than the physical gaps between fibers would suggest.
A surgical mask follows a three-layer design. The outer and inner layers are spunbonded polypropylene, a coarser, more durable fabric that repels water and provides structure. Sandwiched between them is the melt-blown polypropylene filter layer with its denser, finer fiber structure. N95 masks add a fourth layer of modified acrylic that provides extra shape and thickness.
There are three main ways to charge these fibers: corona charging (exposing them to a high-voltage electrical field), friction charging, and induction charging. The charge degrades over time, which is one reason disposable masks lose effectiveness with extended use or washing.
Oil and Fuel Filters
Most standard automotive oil filters use cellulose media, essentially processed wood pulp formed into a pleated paper-like sheet. Cellulose is inexpensive and effective at catching the larger particles that circulate through engine oil. For more demanding applications, manufacturers blend cellulose with synthetic glass fibers. The synthetic fibers add consistency and strength, capture finer contaminants, and allow the filter to last through longer oil change intervals. Premium or high-mileage filters sometimes use fully synthetic media for maximum particle capture.
Fuel filters and cabin air filters follow similar principles, using cellulose, synthetic fiber mats, or a combination, chosen based on the size of particles they need to trap and how long the filter needs to last between replacements.
Laboratory and Industrial Membranes
Filters used in labs and industrial settings are made from specialized plastics chosen for their chemical resistance. PTFE (the same material as Teflon) membranes are hydrophobic, chemically inert, and can withstand a wide temperature range. They’re used for filtering aggressive solvents and chemicals like acetone that would dissolve other filter types.
Polyethersulfone (PES) membranes take the opposite approach. They’re hydrophilic, meaning they naturally attract water, making them ideal for filtering water-based solutions. PES handles high-pH environments well and works with most common solvents, both water-based and organic. Each membrane material has specific chemical incompatibilities, so labs choose their filter type based on exactly what liquid they need to process.
What All Filters Have in Common
Regardless of the application, every filter works through some combination of three mechanisms. Size exclusion physically blocks particles too large to fit through the pores. Adsorption causes contaminants to stick to the filter material’s surface through chemical attraction, which is how activated carbon works. And electrostatic attraction pulls charged particles toward charged fibers, which is the principle behind N95 masks and electret air filters. Most high-performance filters use more than one of these mechanisms at once, which is why the material matters so much. The right fiber diameter, pore size, surface chemistry, and electrical properties all work together to determine what gets through and what doesn’t.