I often see "ppo" when people explain how much fragrance or other additives to use. What does "ppo" mean and how do I use it?
Many ingredients are based on "ppo" which is an abbreviation for "per pound of oils." See this article for more about using "ppo" measurements.
Some suppliers give "IFRA guidelines" for their fragrances. What is IFRA? Why should I care about these guidelines?
IFRA is the acronym for the International Fragrance Association. It is an association that sets voluntary standards with the goal of helping manufacturers "...provide products that are safe for use by the consumer and to the environment..."
There are IFRA guidelines for essential oils (EOs) and fragrance oils (FOs) for use in soap, cosmetics, and other products. You can often find IFRA guidelines on suppliers' websites. IFRA standards are voluntary, however, so suppliers are not obligated to provide IFRA guidelines to their customers.
Nature's Garden (NG) is one of many suppliers who provide IFRA guidelines for their fragrances. Here are the various IFRA categories as described on the Nature's Garden website --
Category 1: Ingredients must be GRAS [Generally Recognized As Safe]. Products in this category include: Lip products & Toys
Category 2: Deodorant & Antiperspirant products
Category 3: Eye products, male facial creams, baby creams, baby lotions, baby oils, products applied to recently shaved skin
Category 4: Products applied to unshaved skin (perfumes), hair styling products, hair sprays, body creams, body oils, lotions (except for baby products), hair deodorant, foot care products
Category 5: Female facial creams, facial make-up, hand cream, facial masks, baby powder/talc, wipes or refreshing tissues for face, neck, hands, body.
Category 6: Ingredients must be GRAS. Products include: mouthwash, toothpaste.
Category 7: Intimate feminine wipes, toilet wipes, baby wipes, insect repellent
Category 8: Make-up removers, nail care, hair dyes
Category 9: Bar soap, bath gels, foams, mousses, salts, oils & other products added to bathwater, body washes, conditioner (rinse off), face cleansers, liquid soap, shampoo of all types, shaving creams of all types, aerosol air freshener sprays
Category 10: Laundry detergents of all types, fabric softeners of all types, household cleaning products, dishwashing detergent, shampoos for pets
Category 11: All non-skin contact, air fresheners, plug-ins, solid substrate, membrane delivery, electrical, potpourri, powders, sachets, incense, lamp rings, reed diffusers, joss & incense sticks, animal sprays & cat litter, candles, deodorizers, maskers, insecticides, scratch and sniff
Here are the IFRA guidelines for a random fragrance, again from the Nature's Garden website --
MAXIMUM USE LEVEL
CATEGORY 1: 0%
CATEGORY 2: 1.1%
CATEGORY 3: 1.1%
CATEGORY 4: 5%
CATEGORY 5: 4%
CATEGORY 6: 0%
CATEGORY 7: 3.9%
CATEGORY 8: 4%
CATEGORY 9: 5%
CATEGORY 10: 14.9%
CATEGORY 11: No Restriction
A fragrance has a IFRA Category 4 limit of 57% and a Category 9 limit of 10%. Why is it okay to use more fragrance in lotion, which is a Cat 4 product that remains on the skin, than in soap, a Cat 9 product that is rinsed off?
Category 4 is for water- and/or alcohol-based products that are applied to unshaved skin and left on the skin.
Category 9 is for rinse-off cleansers and other bath products that contact shaved skin or sensitive skin (face, genitals). This category also includes non-cleanser products used in the genital area (feminine pads) and aerosols (air fresheners) that can be inhaled.
Here's my perception, speaking as a non-expert on IFRA --
The key difference between the two categories is less about the rinse-off or leave-on aspect and more about whether the product is likely to contact sensitive (genital, facial) or irritated (shaved) skin or to be inhaled. Category 9 products are ones that could have a direct entrance into the body through damaged or sensitive skin or via the respiratory system.
A fragrance has a recommended IFRA Category 9 (soap) usage rate of 8%. Does this mean 8% of the total fat weight or 8% of the total soap weight?
Within the handcrafted soap making community, ingredients are often measured on the basis of "ppo" -- in other words based on the weight of fat in a soap batch. I think the reason why soap makers use this basis is because bar soap loses water due to evaporation, while the weight of fat in the soap remains a constant.
Pretty much everyone else in the cosmetics world measures ingredients as a percentage of the total weight of finished product. The IFRA guidelines are on this basis.
The amount of fragrance you use in your soap should be within the IFRA limits at the time the consumer receives the soap. The weight of ingredients to make a batch of soap is always going to be more than the weight of cured soap that the consumer receives.
If you calculate fragrance based on the starting batch weight and you use the maximum dosage that IFRA recommends, the fragrance in the cured soap could exceed the IFRA safety limit when a consumer receives it.
I estimate there is roughly a 10% loss of weight by the time a soap bar is cured (4-8 weeks after it is cut). If you want to calculate the amount of fragrance based on the final cured weight, you should include that weight loss into your calculations.
I suspect this weight loss during cure is one reason why many ingredients are based on the total fat weight (ppo) rather than the final cured weight. The weight of oils is a constant value and easy to use in calculations. There is nothing wrong with calculating the amount of fragrance on the total oil weight -- it is far better to err on the side of using less fragrance rather than risk overdoing it.
The IFRA guideline for Category 9 (soap) recommends a maximum of 23% for a particular fragrance. Does this mean I can use up to 23% of this fragrance in my soap?
Not necessarily. An IFRA guideline is only concerned with human and environmental safety. For example, some fragrances have a Category 9 limit of 100%. Does it make sense to make a "soap" that is 100% fragrance?
I do not use more fragrance than the IFRA guideline, because I want my soap and lotion to be safe for people to use. On the other hand, I may use less fragrance than what the IFRA guideline says, becaue I also want my soap to perform well.
There is a practical limit for how much scent you can add to a soap and not run into problems. Too much fragrance can weep out of bar soap, cause the soap to be soft, possibly reduce the lather or longevity, or create other undesirable problems.
To encourage reasonable use, some suppliers, such as Nature's Garden, make their own recommendations for maxiumum amounts for various products. For the random fragrance given in the example above, Nature's Garden recommends this --
Bath oils, soaps, bath gels: Maximum use 5%
This happens to be the same percentage as the IFRA guideline for this fragrance. For a fragrance with a 23% IFRA guideline, I'd probably still use about 5% of this fragrance to ensure the soap is skin safe and also performs well.
I normally scent my soap at 5% to 6%, assuming the IFRA guideline allows. On occasion, I will use more if I am scenting only part of the soap, but I limit the rate to 8% maximum in the scented portion. The few times I have added fragrance above 8%, the fragrance has weeped out of the soap with unattractive results.
Other soap makers tell me they do not have trouble when they add fragrance at higher percentages, again assuming the IFRA guideline allows, so be aware this 8% limit is my rule of thumb, not anyone else's.
I want to use a fragrance with an IFRA guideline of 0.8%. At that rate, I can barely smell it in my soap, but I love, love, love this scent. Can I use more than what the guideline says?
There are no IFRA police out there to stop a person from using more fragrance than the guideline recommends. I personally follow IFRA guidelines, however, and advise others to do the same.
Some fragrances contain ingredients that are allergens or sensitizers and are safe for use on skin only at very low doses. The IFRA guidelines take these safety hazards into account.
Adding enough fragrance until the soap smells good, with no thought to the safety considerations, is not a responsible thing to do. Unless you are willing to dig deep into the toxicology of the various ingredients in a fragrance, I recommend the safe, easy solution of following the IFRA guidelines.
Another one of my rules of thumb -- When I see a fragrance with an IFRA guideline that is below about 6%, I will not buy that particular fragrance. I like to make strongly scented soap, but I also want my soap to be as safe as possible. Fragrances with low IFRA limits do not give me enough peace of mind, so I do not use them, no matter how amazing they smell.
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