There are no official guidelines to assist in answering this question. I will search for relevant research papers instead.
From the Research
Benefits of Skin Toner
- Skin toner can be beneficial in reducing oiliness of the skin, as shown in a study where guava toner significantly reduced oiliness of the forehead and nose compared to a base toner 1.
- Using a skin care routine that includes a toner can improve superficial hydration, deep hydration, skin roughness, mean pore area, melanin heterogeneity, and crow's feet wrinkle depth compared to a simple routine with only a cleanser and day cream 2.
- Toner can be used in combination with other moisturizers, such as cream, to achieve greater hydration, especially for normal skin 3.
- Repeated application of toner can be efficacious for skin hydration, especially when used in combination with cream 3.
Ingredients in Skin Toner
- Guava extract has been shown to have anti-sebum efficacy and can be used in skin toners to reduce oiliness 1.
- Rice-derived ingredients, such as phenolic compounds and betaine, have been found to have anti-aging, anti-inflammatory, whitening, photoprotective, and moisturizing effects, and can be used in skin toners 4.
- Niacinamide, a component of some skin toners, has been shown to have a stabilizing effect on epidermal barrier function, improve moisture content, and increase protein synthesis, making it a suitable component for cosmetic products 5.