Filters are remarkable pieces of engineering. A quality H13 HEPA filter captures particles down to 0.3 microns at 99.95% efficiency. A reverse osmosis membrane blocks ions measured in angstroms. Meanwhile, activated carbon adsorbs the molecules behind taste, odor, and chemical toxicity.
But no filter removes everything. Understanding what falls through the cracks isn’t pessimism—it’s the foundation of building a filtration system that actually works for your specific situation, rather than one that gives you false confidence.
Here’s an honest look at what even the best filter cartridges struggle with or cannot meaningfully address.
The Foundational Principle: Every Filter Has a Design Limit
Filter cartridges work through one or more physical or chemical mechanisms: mechanical size exclusion, adsorption, ion exchange, or redox reactions. Each mechanism operates within a defined range—and contaminants outside that range pass through.
The complete version of “this filter removes 99.97% of particles” is: of particles at or above a specific size, under specific flow rate conditions, within the rated lifespan of the media, when the cartridge is properly installed. Change any of those variables and performance changes with it.
What HEPA Filters Cannot Remove
HEPA filters are the gold standard for airborne particle capture. H13-grade HEPA removes 99.95% of particles at 0.3 microns. H14-grade reaches 99.995%. But the limitations are real and worth knowing.
Gases and Volatile Organic Compounds
HEPA captures particles. Gases are not particles. Formaldehyde, benzene, nitrogen dioxide, ozone, carbon monoxide—none of these are meaningfully stopped by HEPA filtration. They move straight through the fiber matrix.
This is why the combination of HEPA plus activated carbon is the industry standard for serious air purification. Carbon handles gases and odors; HEPA handles particles. You need both for comprehensive coverage.
Common indoor VOC sources include: new furniture, flooring, paint, cleaning products, scented candles, cooking at high temperatures, and building materials that off-gas for months or years after installation.
Ultrafine and Nanoscale Particles
Here’s a counterintuitive fact about HEPA filters: they’re actually least efficient at exactly 0.3 microns—which is precisely why that’s the industry-standard test size. It represents the most penetrating particle size for fibrous filter media.
Below 0.1 microns, HEPA efficiency increases again because extremely small particles move erratically due to Brownian motion and collide with filter fibers more frequently than larger particles do. However, nanoparticles in the sub-0.01 micron range begin to behave more like gas molecules and may not be captured with the same reliability. This remains an active area of filtration research and is relevant to discussions about nanoplastics and combustion ultrafines.
Carbon Dioxide
CO2 is produced by every person in an enclosed space. It’s a gas, not a particle, and no consumer air purifier—regardless of price or filter quality—meaningfully reduces CO2. The only solution is ventilation: exchanging indoor air with outdoor air. This is one of the strongest practical arguments for combining window ventilation with air purifier use rather than relying exclusively on either. For a fuller treatment of this, see our guide on opening windows versus air purifiers.
Mold Colonies and Surface Contamination
Air purifiers capture airborne mold spores effectively. But if mold has already colonized a wall surface, ceiling tile, carpet backing, or HVAC system component, filtering the air addresses the symptom while the source continues to produce new spores. Surface mold requires physical remediation and elimination of the moisture source. No air filter resolves an active mold infestation.
What Activated Carbon Cannot Remove
Activated carbon is the most versatile filtration media available for both air and water applications, but it has clear and well-documented limits.
Heavy Metals in Solution
Standard granular activated carbon does not reliably remove dissolved heavy metals like lead, arsenic, or cadmium from water. Metal ions are small and carry charges that don’t adsorb to standard carbon surfaces effectively. Claims of “reduces heavy metals” on activated carbon-only filters should be scrutinized carefully and verified with independent NSF certification data that specifies which metals were tested and at what reduction rate.
Specialty media—like KDF copper-zinc alloy combined with carbon block, or carbon with chelation resin—performs substantially better. But that’s a different product than plain activated carbon. See our article on whether filter cartridges can remove heavy metals for a full technology-by-technology breakdown.
Nitrates and Fluoride
Both nitrates and fluoride are anions. They pass through activated carbon with minimal interaction because carbon’s adsorption mechanism doesn’t target these ions effectively. Removing nitrates requires ion exchange resins; removing fluoride requires activated alumina media, bone char, or a reverse osmosis membrane. This is a common source of confusion for consumers who purchase carbon filters expecting comprehensive water treatment.
Carbon Saturation and Breakthrough
Once activated carbon reaches its adsorption capacity, it stops removing contaminants—and can begin releasing previously captured substances back into the filtered water or air. This is called breakthrough, and it’s the primary reason filter replacement schedules exist.
What Reverse Osmosis Membranes Cannot Remove
RO is the most thorough filtration technology available for residential and commercial water treatment, but even it has meaningful gaps.
Dissolved Gases
Gases like radon, hydrogen sulfide, carbon dioxide, and certain volatile organic compounds with low molecular weight can pass through RO membranes. Because RO membranes target dissolved ionic species, small-molecule gases behave differently and are not reliably rejected.
This is why carbon pre-filtration is essential in multi-stage RO systems—not optional. Carbon handles gases and chlorine; the RO membrane handles dissolved solids and heavy metals.
Certain Pesticides and Herbicides
Some low-molecular-weight organic compounds aren’t blocked reliably by RO membranes alone. Atrazine, for example, can partially pass through standard membranes depending on concentration and operating conditions. Combined systems—carbon pre-filter plus RO membrane—address this more comprehensively than either technology alone.
Biological Regrowth Downstream of the Membrane
RO membranes are highly effective barriers. But the purified water that exits the membrane is stored in a pressurized tank before reaching the tap in most traditional under-sink systems. If that storage tank isn’t maintained and periodically sanitized, bacteria can colonize the tank itself—after the membrane has already done its job. The filter worked; the storage system created a new problem.
UV post-treatment or tankless RO systems address this concern. It’s a reminder that filtration effectiveness depends on the entire system, not just the membrane cartridge.
What Ultrafiltration Membranes Cannot Remove
Ultrafiltration is an effective biological barrier with limited chemical filtration capability:
- Dissolved heavy metals: Pass through freely—UF pores are far larger than metal ions
- Fluoride and nitrates: Size exclusion or adsorption methods cannot remove
- Chlorine and chloramines: Not removed; a pre-carbon stage is needed
- Total dissolved solids: Not reduced; minerals pass through unaffected
UF is best understood as a biological purification stage, not a comprehensive water treatment solution. It excels at what it’s designed for—and falls short where it was never intended to reach.
Contaminant Categories That Challenge All Standard Filtration
Microplastics and Nanoplastics
Microplastics are increasingly detected in drinking water, indoor air, and even human tissue samples. RO membranes and UF membranes can remove most microplastics by size exclusion. HEPA filters capture airborne plastic particulates. However, nanoplastics—sub-micron plastic fragments—are a newer area of concern that standard filtration may not fully address. The science is evolving faster than the testing standards.
Pharmaceutical Compounds and Endocrine Disruptors
Trace pharmaceuticals—antibiotics, synthetic hormones, antidepressants, and anti-inflammatories—are present in many municipal water supplies at very low concentrations, largely as a consequence of human metabolism and disposal practices. RO membranes reduce most, but not all, pharmaceutical compounds. Activated carbon contributes additional reduction. No standard residential filter eliminates all pharmaceutical compounds at all concentrations under all operating conditions.
PFAS
Per- and polyfluoroalkyl substances are persistent synthetic compounds with documented health associations. They’re found in firefighting foam, food packaging, non-stick cookware, and many industrial applications. High-quality granular activated carbon and RO membranes reduce PFAS at varying efficiency rates, but specific PFAS compounds differ significantly in their interaction with different media. Specialized ion exchange resins designed for PFAS capture perform better than standard carbon for this class of compounds. Regulatory standards for PFAS in drinking water are tightening in multiple countries, making this an active area of filter development.
Radon
Radon dissolved in water requires granular activated carbon in a dedicated whole-house treatment system—not a standard point-of-use under-sink cartridge, which doesn’t have sufficient contact time or media volume to address radon effectively. Radon in indoor air is a building remediation issue. Air purifiers don’t address it; proper building ventilation, sub-slab depressurization systems, and sealing of entry points are the appropriate interventions.
What This Means for Your Filtration Approach
The conclusion isn’t that filtration is ineffective—it’s that no single cartridge or technology is a complete solution for every contaminant. Effective filtration strategy means:
Know what you’re actually filtering against. Get your water tested by a certified laboratory. Check your local AQI data regularly for air quality. Match your filtration investment to your actual exposure profile, not to worst-case marketing scenarios.
Choose the appropriate technology to address the pollutants. For dissolved heavy metals, don’t rely solely on ultrafiltration; conversely, for the release of volatile organic compounds, don’t rely solely on high-efficiency particulate air filters. The technology must be matched to the type of pollutant.
Layer technologies for broader coverage. Multi-stage systems—PP cotton, activated carbon, UF or RO, post-treatment—address a wider range of contaminants than any single filter stage. Each stage protects and complements the next.
Expired filter media will not only stop working, but will also significantly reduce its filtration performance and may even allow contaminants to seep in. Therefore, there is a good reason to develop a replacement schedule regularly.
Address sources, not just symptoms. A filter cannot fix a leaking pipe, a moldy wall, a radon-emitting basement foundation, or a contaminated well. Filtration reduces exposure to contaminants already present in your air or water; it doesn’t eliminate the source.
HIFINE’s Approach to Filtration Engineering
At HIFINE, we manufacture filter cartridges across HEPA, activated carbon, UF membrane, and multi-stage configurations for OEM and ODM customers. Understanding the physical limits of each technology is the starting point for designing filtration products that perform honestly under real-world conditions.
For related reading, our guides on how to choose between PP cotton, carbon, UF, and RO filter cartridges cover complementary ground.
