Beginner perfumer · 55
What percentage is three drops in 10 mL? And why 89% of the corpus looks over the limit
· 21 min read
Somebody on r/DIYfragrance made a wedding perfume out of ylang ylang essential oil and asked whether we overcomplicate things. A reply pointed out that ylang is limited to 0.73 percent of a finished product, and thirty-odd comments of arguing about whether to trust safety guidance followed. The poster stated the actual recipe twice, three drops in 10 mL of alcohol, and nobody converted it. It comes to between 1.09 and 2.17 percent, with the whole range above the line. The body oil they also made runs 0.62 to 1.23 percent, with the line through the middle. The third number is the one worth explaining: in our corpus of 954 formulas the median ylang dose is 2.5 percent and 89 percent of uses exceed 0.73, and those formulas are not in breach.
About a 6 minute read.
Somebody posted that they had made their own perfume for a wedding and got a lot of compliments, and that the recipe was ylang ylang essential oil and lavender in alcohol. Do we overcomplicate things at times, they asked.
One reply produced a number.
"Did you account for the fact that ylang is limited to a max of 0.73% of the final product?" (berael)
The next thirty-odd comments are an argument about whether to trust safety guidance. The poster says they have zero belief in safety guidelines and go by common sense and instinct; the other side says that without research it is not knowledge. Neither moved.
And the poster stated the actual recipe, twice.
"I used three drops essential oil to 10 ml alcohol. I also made an oil using sesame oil to use as a body oil... only 2 drops per 10 ml oil."
Out of 42 replies, nobody converted those three drops into a percentage.
The conversion
The database gives ylang ylang flower oil a specific gravity of 0.948 to 0.968 (I use the midpoint, 0.958), and ethanol 0.772 to 0.786 (I use 0.779).
Essential oil droppers commonly land between 20 and 33 drops per mL, that is 0.03 to 0.05 mL per drop. Since I do not know which he has, I compute the whole range.
Three drops in 10 mL of ethanol, by weight:
| Drop volume | Concentration | Against 0.73% |
|---|---|---|
| 0.03 mL | 1.09% | 1.50x |
| 0.04 mL | 1.45% | 1.99x |
| 0.05 mL | 1.81% | 2.48x |
| 0.06 mL | 2.17% | 2.97x |
The whole range sits above the line. The smallest plausible drop still gives 1.5x.
The conclusion does not require knowing which dropper he has, because every plausible dropper gives the same answer.
The body oil is a different case
Two drops in 10 mL of sesame oil (specific gravity around 0.92):
| Drop volume | Concentration | Against 0.73% |
|---|---|---|
| 0.03 mL | 0.62% | 0.85x |
| 0.04 mL | 0.83% | 1.13x |
| 0.05 mL | 1.03% | 1.41x |
| 0.06 mL | 1.23% | 1.69x |
The line runs through the middle of it.
Somebody in the thread wrote that 2 drops in 10 ml of carrier oil is above the IFRA limit. At a typical drop size they are right, and at a small drop size they are not.
Why a drop is not a unit, and how to measure your own dropper, is in Why formulas should not use drops. One line from it is needed here: Sklubalová and Zatloukal's 2005 paper found a coefficient of variation of only 3.3% on drop volume once conditions were controlled, while dropper tip design, dispensing angle and dispensing rate all had significant effects. A drop is stable for you and not stable across bottles.
What that 89% is about
This is the part the article is really for.
Across 954 published formulas, 159 use ylang ylang, or 16.7%. Median dose 2.500%, 90th percentile 8.000%, maximum 24.82%.
89% of uses exceed 0.73%.
That looks like professional formulas breaking the rule wholesale. They are not.
Those 954 are concentrate formulas, and the 0.73% governs a finished product.
Ylang at 2.5% of a concentrate, made up into a 20% eau de parfum, is 0.5% in the bottle. Under the line.
For 2.5% in a concentrate to exceed 0.73% in the finished product, the concentrate would have to be above 29.2% of the bottle (0.73 ÷ 2.5), which is beyond extrait strength.
Put differently, the corpus median and that limit never conflicted, because they are not measuring the same thing.
The poster's problem was not dosing high, it was skipping a step. Putting essential oil straight into alcohol makes that number the finished-product number. The 2.5% in a professional formula has another dilution after it.
This is the same hole I fell into writing about solvents: the optimum at the concentrate stage and the optimum for a finished product are two different problems. Settle the denominator or the number means nothing.
What the line is about
Ylang's acute toxicity numbers are not alarming: rat oral LD50 greater than 5000 mg/kg, rabbit dermal LD50 greater than 5000 mg/kg.
The restriction is not about acute toxicity. The GHS field on the same record carries H315 (causes skin irritation) and H317 (may cause an allergic skin reaction).
That is what one reply was pointing at.
"Truth is you could develop an allergy and never be able to tolerate using Ylang ylang again. Same with many materials." (_nate69)
There is a number to go with it. When Geier and colleagues analysed the German-language IVDK data in Contact Dermatitis in 2022, twelve essential oils were patch tested in 10,930 dermatitis patients and ylang ylang (I + II) oil had the highest positive rate of the twelve at 3.9%. The detail of that paper, and why it cannot be read as a general-population probability, is in Synthetics give me headaches so I only use naturals.
One angle that article did not cover: the same study found masseurs and cosmeticians over-represented among those who reacted. The difference is frequency of contact and whether you handle neat material, not the material.
Something I missed in an earlier article
Going back through the ylang records for this piece, I found a problem with a figure I used in Material guide: ylang ylang oil.
That article says generic ylang ylang oil has a substantivity of 140 hours and grade I has 319, more than double, and uses it to show that the fractions really do differ.
Both figures are in the database and the conditions match (both neat). The problem is the direction. The trade describes the fractions this way:
"Extra is toppier, more impactful floral and perfumey quality with a somewhat shorter longevity. The third fraction is longer lasting but with slightly less floral impact and twiggy facets." (oval_euonymus)
Fraction I should be the shorter-lasting one, and the database gives it 2.3 times the complete oil's figure.
And grades II and III, the two the trade calls longer lasting, have that field empty.
I cannot tell which side is right. What is certain is that I read the number without checking it against the trade description, and that longevity is the main reason anybody picks a fraction. I have added a correction note to that article.
(This is the second time the substantivity field has caught me out. Last time was the same rose oxide molecule reading 8 hours and 388 hours, where the conditions differed. Here the conditions match and the direction is reversed.)
What you can actually do
- Settle the denominator first. The concentrate percentage times the concentrate's share of the bottle is the finished-product percentage. Whenever you see a limit, ask which layer it governs.
- Measure your dropper once. Twenty drops on a scale, divide by 20, divide by specific gravity. Five minutes.
- Write formulas in percent by weight, not in drops.
- Putting ylang straight into alcohol, 0.73% is about 1.2 drops per 10 mL (working back at 0.05 mL per drop, sg 0.958).
- For higher strength, build a concentrate and then dilute. That is where the corpus median of 2.5% comes from.
- Frequent handling raises the risk. Gloves for neat material are cheap insurance.
What this doesn't establish
- I have not read the IFRA Standard itself. The 0.73% is quoted from the thread, with the same figure on a third-party page. I did not check which product category it belongs to or whether it is current in the 51st Amendment.
- I have not dealt with IFRA's categories at all. Limits differ substantially across product categories and "finished product" means different things in different ones. That is the largest uncertainty in the multiples above.
- The 0.03 to 0.05 mL drop range is a common figure, not a measured one. Sklubalová measured aqueous eye drops; essential oil has different surface tension and viscosity. What that paper supports is that drop volume depends on tip and technique.
- Sesame oil at 0.92 is a generic figure, and this database has no sesame oil record.
- I used the midpoint of ylang's specific gravity range. Using the ends gives 1.79% to 1.83% for the three-drop case; the conclusion is unchanged.
- The 29.2% crossover uses the median of 2.5%, not the actual dilution factor of each of those 159 formulas, which I cannot know.
- That the corpus is concentrate formulas is my inference from their structure (most contain no alcohol, totals are given in parts). A few may be finished-product formulas; I did not check them one by one.
- Geier 2022 is a dermatology clinic population, the 3.9% cannot be read as a general-population sensitisation rate, and they tested the I + II fraction.
- On 140 versus 319 hours, I only established that the conditions match and the direction is opposite to the trade description.