## 2 Materials and Methods
### Assessment of Cytotoxicity--Alamar Blue (AB) and Neutral Red (NR) Uptake Assays  
Cytotoxicity of yarrow extracts was assessed by measuring the reduction of water-soluble and non-toxic to cells sodium salt of resazurin (Alamar blue test, Merck KGaA, Darmstadt, Germany) and by assessing the storage capacity of neutral red dye in lysosomes of tested skin cells in vitro (Neutral red test, Merck KGaA, Darmstadt, Germany) according to the previously described methodology [19]. Cell viability tests after exposure to the tested extract were performed in sterile 96-well flat-bottom plates. The AB assay was performed on black plates, and the NR test was performed on transparent plates (GooglabScientific, Rokocin, Poland). After 24 h from the cells being plated, the cells were treated with yarrow extracts in the concentration range of 0.01-10.0% for the next 24 h. Dilutions of the tested extract were prepared in DMEM culture medium. Control cells were cells (both HaCaT and HDF) cultured in DMEM (without the addition of individual concentrations of extract). In the case of the AB test, after 24 h of incubation, the extracts dissolved in DMEM were aspirated, and 60 uM of resazurin solution were added to the wells. The cells were incubated with the dye for 2 h at 37 \({}^{\circ}\)C. In the NR assay, the individual dilution of the extract was replaced with a 40 ug/mL neutral red solution, and the cells were incubated for 2 h at 37 \({}^{\circ}\)C. After this time, the cells were washed twice with phosphate-buffered saline (PBS, Genos, Lodz, Poland), and then a destaining buffer (C\({}_{2}\)H\({}_{5}\)OH/CH\({}_{3}\)COOH/H\({}_{2}\)O, 50.0%/1.0%/49.0%) was added. The prepared plates were shaken for 15 min at room temperature. Then, the fluorescence of cells in individual wells (AB test) was measured at 570 nm using a microplate reader (ThermoFisher Scientific, Waltham, MA, USA), and the absorbance of samples (NR test) was measured at 540 nm. The control cells were HDF and HaCaT cells grown separately in DMEM medium without the addition of extract. The viability of these cells was assumed as 100%. Cytotoxicity studies were conducted in three independent experiments, in which each extract concentration was tested in triplicate.  
Evaluation of Intracellular Reactive Oxygen Species (ROS) Levels in Skin Cells Without and After UVB Radiation Exposure  
To assess the ability of the tested yarrow extract to reduce intracellular production of ROS in human skin cells in vitro, the fluorogenic probe 2\({}^{\prime}\),7\({}^{\prime}\)-dichlorohydroflouresceindiacetate (H\({}_{2}\)DCFDA) (Thermo Fisher Scientific, Waltham, MA, USA) was used according to [PERSON] et al. [20]. First, HaCaT and HDF cells were seeded separately on two black 96-well plates and incubated for 24 h. Then, the culture medium was replaced with individual concentrations of the tested extract (from 0.01 to 10.0%) prepared in DMEM medium (VWR International, Radnor, PA, USA) and incubated for another 24 h. After incubation, the plates were aspirated and the cells were washed twice in sterile PBS. Cells from one plate were additionally exposed to UVB radiation (0.5 J/cm\({}^{2}\)) in a Crosslinker UVP CL-3000M (Analytik Jena, Jena, Germany). After UVB irradiation of the cells, the PBS was aspirated from both plates and replaced with 200 uL of a 10 uM H\({}_{2}\)DCFDA probe solution dissolved in DMEM without FBS. In both irradiated and non-irradiated cells, the control cells were fibroblasts and keratinocytes untreated with yarrow extract. DCF fluorescence was measured using a microplate reader (FilterMax F5, Thermo Fisher Scientific, Waltham, MA, USA) with excitation and emission spectra peaking at 485 and 535 nm, respectively.