Indoor Air Quality and Ventilation Guide for Rental Homes—Protecting Your Health from CO2, Pollen, and PM2.5

"Since I've been working from home more, I have headaches," "I feel drowsy for no clear reason," "I can't seem to concentrate"—these complaints may stem from your room's air environment. The WHO (World Health Organization) lists indoor air pollution as a major health risk, and the air quality in your home—where you spend much of your day—is extremely important.

In particular, modern rental apartments with high airtightness are prone to the following problems when ventilation is inadequate:

• •Rising CO2 (carbon dioxide) levels: causes reduced focus and mental clarity, headaches, and drowsiness
• •Buildup of volatile organic compounds (VOCs): emitted from new building materials, furniture, and adhesives, contributing to sick building syndrome
• •Mold and dust mite growth from excess humidity: worsens allergies and asthma
• •Infiltration of pollen and PM2.5: becomes problematic during certain seasons and in certain regions

This guide explains practical air quality improvement methods you can implement in your rental home. If you'd like more detailed information on moisture management, also refer to our Condensation and Mold Prevention Guide.

CO2 Levels and Ventilation Basics

CO2 (carbon dioxide) accumulates in your room just from breathing. While outdoor CO2 levels are around 400 ppm (0.04%), indoor levels can reach 1,000–2,000 ppm or higher when ventilation is inadequate.

CO2 Levels and Health Effects:

In a one-room rental apartment (about 20 square meters) with a single occupant working from home with windows closed, CO2 levels can reach 1,500–2,000 ppm within just a few hours.

The ventilation standard is to replace at least 0.5 times the room's air volume per hour (per Ministry of Health, Labour and Welfare guidelines).

How to Properly Use 24-Hour Ventilation Systems

Buildings constructed after 2003 are required to install "24-hour ventilation systems." While most rental apartments have this feature, many residents turn it off due to noise or cold concerns.

The 24-hour ventilation system is managed by the resident. Check the following:

Location of the power switch: Usually found in the bathroom, toilet, or hallway. If it's off when you move in, turn it on first.

Filter cleaning: Dust buildup in ventilation ports (intake and exhaust) significantly reduces ventilation efficiency. Remove and vacuum or rinse the filter every 3–6 months. Filter cleaning is the tenant's responsibility in most rental properties.

Opening and closing the intake vent: Some residents close the intake vent (the slot where outside air enters) during cold seasons, but this defeats ventilation. If outside air is cold, set the system to "low" mode or position the intake to avoid direct drafts. If you're struggling with heating costs versus ventilation, see our Winter Energy Savings and Cold Weather Preparation Guide.

Effective Window Ventilation Methods

Combining the 24-hour system with natural window ventilation significantly improves air circulation.

Cross-ventilation (diagonal window opening): Opening two or more windows diagonally creates an air flow path for quick, efficient ventilation. If your one-room unit has only one window, even cracking the front door helps.

Ventilate 2–3 times daily for 5–10 minutes: Concentrated short bursts when you wake up, after meals, or before bed are most effective. Brief, vigorous ventilation expels CO2 more efficiently than leaving windows open all day.

Always use exhaust fans while cooking and after bathing: Run your kitchen exhaust fan during and for 10–15 minutes after cooking to expel cooking fumes, water vapor, and carbon monoxide (if using a gas stove). Run your bathroom exhaust fan or dryer for at least 30 minutes after bathing.

Dealing with Pollen and PM2.5 Seasons

Spring (cedar and cypress pollen) and Asian dust/PM2.5 seasons make open-window ventilation tricky.

Basic strategy: On days with poor air quality, rely on your 24-hour system instead of opening windows.

Intake vent filter replacement: Commercial intake vent filter sheets significantly reduce pollen and PM2.5 infiltration. Available at 100-yen shops and home centers (replace every 1–2 months).

Using an air purifier: If you have pollen allergies, a HEPA-filter air purifier is highly effective. Choose a model that handles PM2.5 as well. Purifiers are allowed in rentals and can be taken with you when you move, so no landlord approval is needed.

Drying laundry indoors: During pollen and PM2.5 seasons, outdoor drying brings allergens inside. Use your bathroom dryer or a dehumidifier to switch to indoor drying.

Preventing Mold and Dust Mites Through Humidity Control

When indoor humidity exceeds 60%, mold and dust mites thrive. Aim for 40–60% humidity.

In rentals, winter condensation (water droplets on windows and walls) is especially prone to mold. Ventilation to expel moisture is your first line of defense—poor ventilation and mold growth go hand in hand.

Watch out for overusing humidifiers: While many use them for winter dryness, excessive humidity causes mold. Use a humidity meter and aim for 50–55%.

New unit considerations: Newly built or recently renovated units emit VOCs from materials, paint, and adhesives. Ventilate especially vigorously during your first few weeks.

Air Quality Improvement Products for Renters

Here are portable, non-permanent products to improve air quality:

• •CO2 monitor: Displays your room's CO2 level numerically. Available for ¥3,000–8,000. Use it as a signal to ventilate when it exceeds 1,000 ppm.
• •Air purifier: Effective against pollen, PM2.5, and viruses.
• •Humidity meter (thermohygrometer): Essential for humidity control. Available for around ¥1,000.
• •Intake vent filter sheet: Prevents pollen and PM2.5 infiltration.
• •Dehumidifier: Tackles high humidity during rainy seasons and summer. Also prevents mold in closets and storage.

Summary: Making "Air Management a Habit" the Foundation of Healthy Rental Living

For rental homes, the foundation of good air quality is: keeping your 24-hour system running, opening windows regularly, and cleaning filters periodically.

Just by placing a CO2 monitor and making your levels visible, you'll know exactly when to ventilate and be motivated to act. For those working from home or spending long hours indoors, investing in air quality pays off in health and productivity. Start by checking whether your 24-hour system's switch is already on.