Last updated: June 12, 2026
In today’s homes, we increasingly talk about comfort, energy efficiency, and energy conservation. However, we rarely stop to think about how the air we breathe affects our daily well-being. Scientific studies show that indoor air can be up to five times more polluted than outdoor air, but in Lithuania—especially during the heating season— the concentration of fine particulate matter in the air often exceeds World Health Organization (WHO) recommendations.
Recuperation systems help reduce these negative effects— but only if the recuperation filters are working effectively. Clogged filters not only lose their protective function, but can also pose a health risk.
Types of air filters: What do G4, M5, F7, and activated carbon filters capture?
When choosing filters, it is important to understand what they protect against and what types of particles they can trap. Indoor air quality is affected by particles of various sizes—ranging from coarse dust to microscopic bacteria and smog compounds. Each class of filters serves a different purpose.
G4 filters are primary filters that trap coarse dust, hair, insects, and larger pollen particles. They effectively protect the heat recovery unit itself, but provide very limited protection against fine particles that are hazardous to health.
M5 filters are one of the most commonly used standards in Lithuanian homes. They trap most pollen, dust mites, and mold spores, and effectively filter particles that are approximately 1–2.5 microns in size — it is precisely these particles that most commonly cause allergies and respiratory irritation.
F7 filters are high-efficiency fine particle filters capable of capturing up to 90–95% of particles as small as 0.3 microns. For comparison, many bacteria, such as staphylococci and streptococci, as well as smog particles, range in size from 0.3 to 2 microns. That is why F7 filters are particularly recommended for people who live in cities, near busy roads, or suffer from allergies.
Activated carbon filters provide additional protection against odors and gases. They absorb cigarette smoke, cooking odors, combustion byproducts, and industrial gases. This is especially important for residents of apartment buildings, where cigarette smoke from neighbors or airborne viruses can enter through ventilation systems or open windows. Activated carbon also traps chemical compounds that contribute to smog formation.
Why are dirty filters dangerous? 5 scientifically proven reasons
1. The most dangerous fine particles end up indoors
The smallest particles— invisible to the naked eye—are the most harmful. In Lithuania, 2023. In 2019, levels exceeded WHO recommendations in all major cities. These particles penetrate deep into the lungs and even enter the bloodstream, increasing the risk of cardiovascular and respiratory diseases. A dirty filter can no longer trap them effectively— the airflow weakens, and the contaminants simply bypass the filter material.
2. During the heating season, smog and smoke particles enter homes
Lithuanian winters are characterized by higher levels of smog, caused by the burning of fuel, poor air circulation, and stagnant air. Smoke particles are often smaller than 1 micron, so only M5 or F7 class filters can effectively capture them. An old filter can lose up to half of its filtration efficiency, allowing smog to easily enter living spaces.
3. Clogged filters become breeding grounds for bacteria and mold
Damp and dusty filters create ideal conditions for the growth of microorganisms. If filters are not changed for more than 3–6 months, the heat recovery system may begin to blow unpleasant odors, bacterial particles, or mold spores into the rooms . This is particularly dangerous for young children, the elderly, and people with weakened immune systems.
4. Higher risk of inhaling viruses and odors from neighbors in apartment buildings
Viruses themselves are extremely small, but they often travel by attaching themselves to other particles, which F7 filters can capture with up to 95% efficiency. This reduces the risk in situations where a neighbor is ventilating their home while sick or when smoke and odors are circulating in the air. Activated carbon ensures that odors and gases do not spread between apartments.
5. Increased risk of allergies and asthma flare-ups
Pollen and mold spores are among the most common allergens. Studies show that opening windows can allow up to 70% of outdoor pollen to enter indoor spaces. When filters become worn out and allow more particles to pass through, people with allergies feel the effects immediately— runny nose, coughing, sneezing, and difficulty breathing.
A heat recovery system can be an effective tool for creating a healthier indoor climate—but only if the filters are properly maintained. Contaminated filters not only fail to protect against air pollution, but can actually increase it. That is why experts recommend replacing heat recovery filters every 3 to 6 months, and activated carbon filters even more frequently.
If you’re looking for reliable, certified heat recovery filters, Filtru pasaule offers a wide range of filters, home delivery, and personalized consultations tailored to your system model.
