Analysis of the Formula and Potential Causes

Your formula contains several active ingredients, and the reactions observed (itching, redness, rash) are common signs of irritation. The most likely culprits are:

• Vitamin B3 (Niacinamide) at 4%: While beneficial, Niacinamide can cause flushing, redness, and itching, especially in sensitive individuals or if the grade contains higher levels of nicotinic acid. Although 4% is not excessively high, it can still be irritating for some.
• Pitera (Galactomyces Ferment Filtrate) at 10%: Fermented ingredients can sometimes trigger reactions like breakouts or irritation in sensitive skin. Since this ingredient was not sourced from MySkinRecipes, its quality and potential impurities are unknown factors that could contribute to irritation.
• Arbutin (Alpha-Arbutin) at 1.25%: Alpha-Arbutin is generally well-tolerated, but individual sensitivity is possible. Like Pitera, the quality of this ingredient is unknown as it was not purchased from MySkinRecipes, making it a potential source of irritation.
• Nature Preserve at 1.25%: Preservatives are necessary but are also common causes of contact dermatitis (redness, itching, rash). The specific type of preservative blend in "Nature Preserve" and its concentration (1.25% is relatively high for some preservatives) could be irritating for some users.
• Combination of Active Ingredients: The formula contains multiple active ingredients aimed at various benefits (lifting, brightening, moisturizing). The combined effect of these actives, even at seemingly moderate concentrations, might be too stimulating or irritating for sensitive skin types.

Other ingredients like Pullulan, N-Acetylglucosamine, Natural Moisturizing Amino acid, Aloe Vera Extract, and Vitamin B6 are generally less likely to cause significant irritation at these concentrations, but individual allergies or sensitivities are always a possibility.

How to Troubleshoot and Resolve the Issue

The best approach is to systematically identify which ingredient(s) are causing the problem.

1. Prioritize Ingredients Not Sourced from MySkinRecipes: Since the quality of Arbutin and Pitera from other sources cannot be guaranteed, these are strong suspects.
   • Try making a test batch of your น้ำตบ without the Arbutin and Pitera. Have the affected individuals test this modified formula (perhaps on a small area first). If the reactions stop, then one or both of these ingredients were the cause.
   • If removing them resolves the issue, you could try adding them back one at a time in separate test batches to pinpoint which one is the primary irritant. Consider sourcing these ingredients from a trusted supplier like MySkinRecipes if available.
2. Adjust Niacinamide Concentration: If removing Arbutin and Pitera doesn't solve the problem, try reducing the concentration of Vitamin B3 (Niacinamide). Prepare a test batch with a lower concentration, such as 2% or even 1%, and see if the reaction improves.
3. Evaluate the Preservative: Check the recommended maximum usage rate for the specific "Nature Preserve" product you used. Ensure you are within the recommended guidelines. If the concentration is appropriate, consider if a different type of preservative might be less irritating for these individuals.
4. Simplify the Formula: If the above steps don't isolate the issue, try making a very basic version of the น้ำตบ with just distilled water, a simple humectant (like the Natural Moisturizing Amino acids), and the preservative. If this base is tolerated, gradually add back other ingredients one or a few at a time in new test batches to see which addition triggers the reaction.
5. Check Lot Numbers: For the ingredients purchased from MySkinRecipes, follow the staff's advice and provide the lot numbers so they can check for any potential quality issues with specific batches.
6. Consider Individual Sensitivity: It's important to remember that not all cosmetic products are suitable for everyone. Some individuals have more sensitive skin and may react to ingredients that are well-tolerated by others.

By systematically testing variations of your formula, you can narrow down the potential cause of the irritation and adjust the formula to be more suitable for sensitive skin.

If the skin reactions are severe or do not improve after adjusting the formula, advise the individuals to discontinue use and consult a dermatologist.