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uHoo Proprietary Health Index · Research-backed

Flu season is not just outside. It lives in your air.

The flu spreads through the air in rooms that are too cold, too dry, and too poorly ventilated. Those are conditions you can measure. The uHoo Influenza Index scores them daily so you know when your indoor air is helping the virus and when it is not.

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✓ Built on published science ✓ Real-time detection and alerts ✓ Covers 9 parameters including PM2.5, NO₂, and ozone
Influenza Index · Live reading
2
Low risk · Good conditions
Humidity50%
Temperature21°C
CO₂780 ppm
PM2.58 µg/m³
Air unfavourable for flu transmission
10
Point scoring scale
9
Parameters in the model
Daily
Index recalculation
Published
Science-backed methodology
What the Influenza Index measures

Your home either helps the flu survive or it does not. Now you can tell which.

Influenza spreads through aerosols, tiny virus-carrying particles that stay suspended in the air for minutes to hours depending on the conditions. Cold temperatures preserve the virus. Low humidity keeps the aerosols floating. Poor ventilation lets them build up. These are not random factors. They are things your indoor air is doing right now.

The uHoo Influenza Index scores those conditions from 1 to 10. It uses 9 parameters: temperature, humidity, CO₂, PM2.5, NO₂, CO, VOCs, ozone, and air pressure. Some of these carry the virus. Others damage your airway and make you more likely to catch it even at lower viral loads.

The full methodology, parameter relationships, and ideal indoor ranges are in the white paper. What the page can tell you is that a score of 2 in a warm, well-humidified, well-ventilated room is genuinely protective. A score of 8 in a cold, dry office in January is not.

Indoor air quality visualization for the uHoo Influenza Index
Scoring system

What your score means and what to do about it.

The index runs from 1 to 10. Every level maps to a specific environmental risk picture and a set of practical actions you can take today.

1-3
Good
Your indoor conditions are working against the flu, not for it.
Temperature, humidity, and ventilation are in a range where influenza aerosols do not stay airborne for long, viral loads dissipate faster, and your respiratory defences are supported. Keep it this way and uHoo will tell you if anything shifts.
Maintain humidity between 40% and 60% Keep temperature around 21°C No immediate action needed
4-6
Mild
Transmission is possible, particularly for people who are already vulnerable.
Some conditions have moved outside the good range. Healthy adults may not notice. Older adults, children, pregnant women, and anyone immunocompromised face a higher chance of catching something in this environment. Now is a good time to improve ventilation or humidity before the score climbs further.
Check humidity and bring it toward 50% Ventilate when outdoor conditions allow Reduce crowding in small rooms
7-8
Bad
Conditions are genuinely helping the virus. Act before someone gets sick.
Environmental factors are now combining in ways that extend how long influenza aerosols stay infectious and how far they travel. If anyone in the space has flu symptoms, they should not be there. Everyone else should take steps to improve conditions now.
Ventilate immediately Run a HEPA air purifier Bring humidity toward 50% Keep the space less crowded
9-10
Severe
High probability of transmission. Urgent action required.
The combination of environmental conditions and personal susceptibility in this space creates a strong likelihood of influenza transmission. If anyone is symptomatic, they need to leave. Address as many parameters as possible straight away. This score should not be left to sit.
Maximum ventilation immediately Run HEPA filtration at high setting Remove symptomatic individuals Reduce occupancy until conditions improve
What drives your score

Nine parameters. The reasons are in the white paper.

The Influenza Index uses your Caeli's 9 sensor readings because each one affects how the flu virus behaves or how your body responds to it. Here is what each one does. The exact thresholds are in the paper.

Temperature icon
Temperature
Influenza viruses are most stable and transmissible at or below 20°C. Cold air also slows droplet evaporation, keeping aerosols suspended and infectious for longer. The paper documents the specific temperature range that balances viral risk with human comfort.
Target range in the white paper ->
Relative humidity icon
Relative Humidity
Humidity has a dual effect. Too low, and respiratory droplets rapidly shrink into aerosols that float for hours. Too high, and some strains retain infectivity longer. The research points to 50% RH as the optimum. Below 40% is where the real risk builds during winter heating season.
Optimal: 40% to 60% RH, target 50%
CO₂ and ventilation icon
CO₂ and Ventilation
CO₂ tells you how stale the air is. When it climbs above 800 ppm, the room is recirculating exhaled air rather than replacing it with fresh outdoor air. In a room with an infected person, that means their aerosols are building up rather than being diluted.
Target: below 800 ppm CO₂
Particulate matter icon
Particulate Matter
Fine particles act as rafts for viral aerosols, keeping them airborne longer and helping them travel further. PM2.5 in particular penetrates deep into the lungs and is the most relevant size fraction for influenza transmission risk in the home environment.
Target: PM2.5 below 15 µg/m³
NO₂ and VOCs icon
NO₂ and VOCs
These gaseous pollutants do not carry the virus, but they damage airway tissue and suppress immune function. A person breathing elevated NO₂ from a gas stove, or VOCs from cleaning products and furniture, has compromised respiratory defences before the virus even arrives.
NO₂ target: below 53 ppb
Ozone icon
Ozone (O₃)
Ozone irritates lung tissue and impairs the respiratory system's ability to clear pathogens. Even at levels below what most people notice, it increases susceptibility to infection. Indoor ozone sources include certain air purifiers marketed as ionisers, a category worth avoiding.
Target: below 50 ppb ozone
Family at home using indoor air quality monitoring
For families at home

When one person gets flu, the air decides what happens next.

Whether a flu brought home by one family member spreads to everyone else is not just luck. It depends on the air in the rooms you share. A well-managed home with humidity around 50% and CO₂ below 800 ppm gives the virus far less to work with. The Influenza Index tells you where you stand, every day of the season.

✓Scores the air in every room with a uHoo device
✓Alerts you when conditions shift toward higher transmission risk
✓Available with Guard as a home add-on
For offices, schools, and healthcare

Flu season costs organisations millions in lost productivity. The air is where it starts.

A cold, dry office in January with CO₂ at 1,400 ppm is close to ideal for influenza. Facilities teams using uHoo Aura can monitor the Influenza Index across every space in real time, respond before conditions become problematic, and demonstrate proactive air quality management to occupants and certification assessors alike.

✓Influenza Index across unlimited rooms, floors, and locations
✓BMS integration for automated ventilation and humidification response
✓Supports WELL and LEED Indoor Environmental Quality requirements
Explore uHoo Aura for businesses ->
Office, school, and healthcare indoor air quality monitoring
The research behind the index

Built on published science. Download the methodology.

The Influenza Index is not a general wellness indicator. It is a scoring model built on specific published science about how temperature, humidity, particulates, and gaseous pollutants each affect influenza survival and transmission. The paper covers all of it and is free to registered readers.

White paper · 2025 · 16 pages
Optimizing Indoor Air Quality: A Framework for Influenza Virus Risk Reduction
uHoo Research · PDF · For registered readers

The paper covers the environmental determinants of influenza transmission across three domains: physical conditions (temperature and humidity), air cleanliness (particulate matter), and gaseous pollutants. It documents the exact ideal indoor ranges for the parameters, the scoring framework, and the mitigation measures that have the highest impact on reducing your Influenza Index score.

16 pages. Applies to homes, offices, schools, and healthcare environments. Available to registered readers.

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Common questions

What people ask about the Influenza Index.

How do I get the Influenza Index?+
The Influenza Index is available with Guard (the home health intelligence add-on) and uHoo Aura (for business). For home users, Guard includes all four proprietary health indices: Mold, Virus, Influenza, and Pollen, plus Discomfort and AQI. You can add Guard at any time from your dashboard at $79/year. Guard is also free for your first 3 months when you buy Caeli.
How is the Influenza Index different from the Virus Index?+
The Virus Index covers airborne virus transmission broadly, including COVID-19, RSV, rhinovirus, and other respiratory viruses. The Influenza Index is specific to influenza and uses different optimal ranges for temperature and humidity based on influenza-specific research. If you want to know about flu risk in particular, the Influenza Index is the right one to watch during winter months.
Why does winter heating make flu risk worse?+
Two reasons. First, heating dries the air. Relative humidity in a heated home often drops to 20 or 30 percent, well below the 40 to 60 percent range where influenza aerosols lose viability fastest. Second, we close windows in winter, which lets CO₂ and viral aerosols build up rather than being diluted by fresh air. These two factors together are a large part of why flu season coincides with winter, not just because the virus prefers cold temperatures outside, but because heated indoor air creates near-perfect conditions for transmission inside.
Does the index detect the influenza virus directly?+
No. The Influenza Index measures the indoor air conditions that determine how long the flu virus survives and how efficiently it spreads if someone in the space is infected. A low score means current conditions are unfavourable for influenza transmission. Whether the virus is actually present depends on the people in the space, not the air quality alone.
What is the single best thing I can do to lower my score in winter?+
Add a humidifier. In heated winter homes, relative humidity is almost always too low, and getting it to 50 percent is the highest-impact single change you can make. The research is consistent: viral survival is shortest near 50% RH, and your own mucosal defences work better in properly humidified air. After that, improving ventilation to keep CO₂ below 800 ppm is the next most effective action. The white paper covers both in detail.
Does the index work year-round or only in flu season?+
Year-round. Influenza circulates at lower levels outside the traditional winter season, and the indoor conditions that favour transmission exist whenever heating, poor ventilation, or low humidity occur. The index is calculated daily from your live sensor data regardless of season, so it reflects your actual indoor risk throughout the year.
The full index suite

Influenza is one of four things the index watches for you.

Guard and uHoo Aura include three other proprietary health indices, each tracking a different invisible indoor air risk.

Mold Index icon
Mold Index
Tracks humidity, temperature, and air circulation to score the conditions that allow mold to grow. Catches risk early, when the fix is opening a window rather than calling a remediation company.
Learn about the Mold Index ->
Virus Index icon
Virus Index
Some rooms are better at spreading viruses than others. The Virus Index scores your indoor conditions for general airborne virus transmission, covering COVID-19, RSV, rhinovirus, and more.
Learn about the Virus Index ->
Pollen Index icon
Pollen Index
Pollen season does not end at your front door. The Pollen Index tracks how indoor conditions affect pollen persistence, fragmentation, and allergic risk across every season.
Learn about the Pollen Index ->
Available on Guard and uHoo Aura

Know whether your indoor air is helping the flu or fighting it.

Influenza, Mold, Virus, and Pollen indices updated daily from your live sensor data. For home or for business.

Get Guard uHoo Aura for business ->
Available with Guard ($79/yr) • Free for first 3 months with Caeli • uHoo Aura for business