DIY lemon juice and baking soda

The claim

Lemon juice brightens dark spots; a baking soda paste exfoliates and clears skin.

What the evidence actually shows - lemon

Citrus fruits contain furocoumarins (psoralens). Combined with UVA these produce phytophotodermatitis: a direct, immune-independent phototoxic reaction with erythema, vesicles and prolonged post-inflammatory hyperpigmentation. Case reports document it repeatedly, including a recent lime-triggered case, and note it is frequently misdiagnosed as allergy, burn, cellulitis or abuse.

The outcome is precisely the opposite of the intent. Someone applying lemon to fade a dark patch, then going outside, risks creating a darker and longer-lasting one.

What the evidence actually shows - baking soda

Healthy skin surface pH averages 4.7, and the acidic stratum corneum is required for barrier homeostasis, corneocyte cohesion and antimicrobial defence. Raising skin pH is implicated in barrier dysfunction and disease.

The scale of the insult is easy to underestimate. Even plain tap water at around pH 8 raises skin pH for up to six hours, and alkaline pH 8 to 9 promotes dispersal of the resident flora. A baking soda slurry sits near pH 9 and is applied deliberately and repeatedly. It is also a physical abrasive with irregular crystal edges, so it combines mechanical damage with chemical disruption.

What to do instead

For pigmentation: niacinamide, tranexamic acid, alpha-arbutin, azelaic acid or a retinoid - all with evidence, all with sun protection, which is non-negotiable since none of them work without it. For exfoliation: a formulated AHA or BHA at a controlled pH, which is the entire point of buying one rather than making one.