In short
Air care fragrance is an engineering problem: how to deliver a stated amount of fragrance into a room, over a stated time, with a stable character and acceptable safety. A candle, a reed diffuser, a spray and an automatic device each solve it differently. The Lab's home fragrance guide and candle guide give practical formulas and doses, and this lesson considers why they behave as they do, and how to measure this.
What you will learn
- Compare delivery systems for air care
- Explain how fragrance affects candle burn and safety
- Track an evaporation curve and its change in character
Before this lesson: Shampoo, Shower and Surfactant Systems
Delivery systems
| System | Mechanism | Main questions | IFRA, 2023 |
|---|---|---|---|
| Candle | Heat melts the wax pool; fragrance evaporates | Wax fit, wick, burn safety, soot | 12 |
| Reed diffuser | Capillary rise and evaporation | Solvent, viscosity, flash point, fractional drift | 10A |
| Manual spray | Aerosol or pump burst | Solubility, droplet size, fallout, flammability | 10B |
| Automatic device or plug-in | Timed or heated release | Rate, material compatibility, device fit | 12 |
| Car freshener | Passive evaporation, heat in cabin | High temperatures, plastic compatibility | 12 |
Candles
Fragrance has to dissolve in the wax and stay there, and different waxes hold it differently. Too much, and it can leach to the surface, affect the flame, or create soot. A wick must suit the wax, the fragrance and the vessel, because the fragrance changes the way the fuel burns. Changes in formula therefore require a new burn test, even when the dose stays the same. Consider a burn test as a matter of fire safety. Standards such as ASTM F2417 exist, and your market may have its own rules. Measure the throw properly, using the scent throw guide. Never leave a test candle unattended.
Diffusers and fractional evaporation
A reed diffuser's liquid loses its lighter parts first. So the rate slows and the character shifts over weeks. That's physics you already know from vapor pressure. Solvent choice is a key lever. It sets the evaporation rate, the safety profile and the flash point, which you covered in the flash point study guide. Reporting a diffuser's lifespan without a measured curve is a guess.
A worked curve, with invented numbers
| Day | Weight of liquid (g) | Loss per day (g) | Odor note |
|---|---|---|---|
| 0 | 100.0 | Strong | |
| 7 | 86.0 | 2.0 | Strong, slightly sharp |
| 14 | 76.0 | 1.4 | Moderate |
| 28 | 62.0 | 1.0 | Weak, heavier character |
Here the rate is halved by day 28, and the note changes. You could use that to adjust the solvent, or to state a realistic lifespan.
Sprays and other considerations
- A room spray in water needs a solubilizer, or a spray in alcohol has a flammability issue. See the room spray guide.
- Home fragrance often falls under chemical labeling rules, including CLP in the EU, and not cosmetics rules. See home fragrance labeling.
- Think about indoor air, pets, and people with asthma. High fragrance loads in small rooms aren't automatically a virtue.
- Check compatibility with packaging. Fragrance can attack some plastics.
Practical exercise: A diffuser evaporation curve
- Prepare a small reed diffuser test, following your market's limits and using a solvent you have reviewed on its SDS. Use a stable vessel away from heat and ignition sources.
- Weigh it on your scale, reeds included, then again at set intervals for at least 14 days. Log temperature if you can.
- Have someone assess odor from a set distance at each check, without looking at the previous score.
- Plot weight against time, and calculate the daily loss between points.
- Compare with a second vessel that has a different solvent or number of reeds, changing only one thing.
- Write what you could honestly say about its lifespan.
You have done this well if you have real weights, your odor assessment has been done fairly and your lifespan statement states its conditions.
Common mistakes
- Changing the fragrance and keeping the wick. Retest the burn.
- Claiming "lasts 8 weeks" without a curve. Measure it.
- Using a load because it's what others say. Check limits and burn behavior.
Safety
Candle tests involve open flame. Follow safety practice, keep them away from flammables, attend them at all times and stop if there's excess smoke or flare. Don't test a home-made heating device. Avoid improvising with electrical diffusers.
Key terms
- Cold throw and hot throw
- How strongly a candle smells when unlit and when burning.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.