Antiperspirant vs Deodorant: Chemistry & Active Mechanisms

Sweat, Body Odour, and the Underarm Skin Microbiome

The human underarm contains two distinct classes of sweat gland, each with a different secretory profile and different relevance to malodour. Eccrine glands, distributed across the entire body surface, produce large volumes of hypotonic sweat — primarily water, sodium chloride, and traces of urea and lactate — as a thermoregulatory response to elevated core body temperature. Eccrine sweat itself is virtually odourless. Apocrine glands, concentrated in the axilla, groin, and scalp, secrete a lipid- and protein-rich fluid in response to adrenergic stimulation — emotional stress, fear, sexual arousal — rather than thermal cues. Fresh apocrine secretion is also odourless; the characteristic malodour of underarm perspiration arises entirely from the microbial metabolism of apocrine secretion components on the skin surface.

The underarm microbiome is dominated by Corynebacterium and Staphylococcus species, with the relative balance profoundly influencing individual odour character. Corynebacterium species metabolise apocrine lipid precursors — particularly the odourless conjugated fatty acid (E)-3-methyl-2-hexenoic acid (3M2H), released from its glutamine conjugate by bacterial amino-acylase enzymes — into free 3M2H, which is the primary contributor to the characteristic sharp, acrid note of human axillary odour. Additional volatile malodorants include 3-hydroxy-3-methylhexanoic acid (HMHA), generated from the same lipid pool, and a series of sulfanylalkanols produced by bacterial thioester cleavage. Staphylococcus hominis produces the sulfurous and cumin-like odour notes associated with more intense perspiration malodour. The microbial population also hydrolyses ester-linked precursors to release androgens including androstenone and androstenol, which contribute the musky component. Our guide to cosmetics and personal care formulations covers the full regulatory and technical landscape for the sector in which antiperspirant deodorant chemistry sits.

Aluminium Salt Antiperspirants: Sweat Gland Blocking Mechanism

Aluminium chlorohydrate (ACH, Al₂(OH)₅Cl·2H₂O) is the most widely used antiperspirant active globally, present in the majority of roll-on, cream, and stick antiperspirant products worldwide. As described in the technical literature on aluminium chlorohydrate, its mechanism of action is physicochemical rather than pharmacological: the aluminium cation undergoes hydrolysis at the near-neutral pH of the upper sweat duct to form polymeric aluminium hydroxide species. These polynuclear aluminium cations adsorb electrostatically onto the negatively charged keratin proteins lining the eccrine duct wall, forming a gel-like plug that occludes the duct lumen and mechanically reduces the volume of sweat reaching the skin surface.

The aluminium-zirconium tetrachlorohydrex glycine (Al-Zr) complexes used in "clinical strength" and "prescription strength" antiperspirant products are more substantive than ACH — they exhibit higher affinity for keratin and produce a more robust and longer-lasting occlusion. Glycine is included in these complexes as a solubilising and complexing ligand that stabilises the aluminium-zirconium coordination sphere and improves skin compatibility. Aluminium sesquichlorohydrate, an intermediate hydrolysis product between aluminium chloride and aluminium chlorohydrate, is used in certain aerosol antiperspirant formulations where anhydrous conditions are required for propellant compatibility. All aluminium-based actives must be applied to dry skin for optimal efficacy: the presence of existing sweat dilutes the active concentration before the gel plug forms, significantly reducing antiperspirant performance in the first application cycle.

Deodorant Antibacterial Actives: Mechanisms and Regulatory Status

Deodorant active chemistry encompasses several mechanistically distinct approaches to the same goal — reducing the concentration of volatile malodorous compounds on the axillary skin surface. Unlike antiperspirant actives, which are defined by a single mechanistic class (aluminium salt occlusion), deodorant actives span bactericidal, bacteriostatic, enzyme-inhibitory, and malodour-trapping mechanisms. The regulatory status of these actives differs significantly across markets, and formulator selection must account for both the EU Cosmetics Regulation restrictions and the US FDA cosmetic/drug classification system. As documented in the overview of deodorant science, the distinction between killing odour-causing bacteria and preventing their malodour-producing enzymatic activity is commercially important — the latter approach allows non-bactericidal claims in markets where bactericidal claims trigger drug classification.

The most commercially significant deodorant actives currently in use are: triethyl citrate (TEC), which inhibits bacterial lipase activity — preventing the enzymatic hydrolysis of odourless ester precursors to free malodorous fatty acids — without significant bactericidal action; zinc ricinoleate, a zinc salt of ricinoleic acid that physically traps and absorbs volatile malodorous molecules including 3M2H and sulfurous compounds via coordination chemistry; silver-based actives (silver citrate, silver zeolite, silver-PCA), which release silver ions that inhibit bacterial metalloenzymes including sulfhydryl-dependent enzymes critical to malodour production; and ethanol at concentrations above 60%, which denatures bacterial surface proteins and is the primary active in alcohol aerosol spray deodorants. Chlorhexidine, a broad-spectrum biguanide biocide active against both Gram-positive and Gram-negative organisms, is used in clinical-grade deodorant formulations — particularly in medical settings for patients with bromhidrosis. Triclosan, historically one of the most effective deodorant actives, is prohibited as a preservative in rinse-off cosmetics in the EU under Regulation 1223/2009 Annex II and subject to ongoing SCCS review for leave-on applications; new EU-compliant deodorant formulations have moved away from triclosan to the alternatives described above. For context on how these functional ingredients relate to the broader personal care chemistry landscape, our article on shampoo formulation and surfactant selection covers the parallel surfactant-based delivery systems used in personal care products.

Antiperspirant Deodorant Active Classes: Performance Comparison

The table below compares the principal active systems available to formulators developing products across the antiperspirant and deodorant spectrum, from maximum-efficacy clinical-strength antiperspirants to natural-positioned deodorant formulations. Selection criteria for any given product must integrate efficacy requirement, skin compatibility profile, regulatory classification in the target market, cost in use, and compatibility with the chosen delivery format (aerosol, roll-on, stick, gel, or cream).

Roll-On Deodorant Formulation: Vehicle Design and Delivery

The roll-on is the dominant delivery format for both antiperspirant and deodorant products in global markets, and its formulation presents distinct challenges compared to aerosol and stick formats. A roll-on antiperspirant typically consists of an aqueous suspension or solution of aluminium chlorohydrate at the required active concentration, formulated in a vehicle designed to deliver a uniform film to the underarm skin through the ball applicator mechanism. The vehicle must be low enough in viscosity to flow freely from the reservoir through the ball at ambient temperature while remaining stable against sedimentation or phase separation during shelf life. Aluminium chlorohydrate in water at concentrations required for efficacy tends to form dispersed colloidal particles — not a true solution — making suspension stability a formulation design priority. Hydroxyethyl cellulose, xanthan gum, and carbomer-based thickeners are used to provide just enough viscosity to maintain dispersion without impeding roll-on application.

The skin feel and drying performance of the roll-on vehicle are formulated through the selection of co-solvents and film formers. Ethanol is widely used in roll-on antiperspirants as a fast-drying co-solvent that promotes rapid film deposition and reduces the wet feel associated with purely aqueous formulations — it also provides a secondary bacteriostatic contribution to the deodorant performance. Emollient esters (isopropyl myristate, isopropyl palmitate) at low levels improve skin slip and reduce the irritation potential of high-ACH concentrations. Silicone-based emollients (cyclomethicone, dimethicone) are used in premium roll-on formulations for their non-greasy dry skin feel and their compatibility with anhydrous formulation variants. Fragrance in roll-on deodorants serves both as a masking agent for residual axillary malodour and as a key purchase driver — fragrance blends for underarm products must be selected against IFRA guidelines for leave-on products applied to the axilla, with particular attention to sensitisation risk given the repeated daily application to a skin area with high occlusion potential. Our article on emulsion science in cosmetics covers the emulsification and rheology principles that underpin many of the same vehicle formulation decisions required in cream and emulsion deodorant formats.

Natural Deodorant Ingredients and Regulatory Considerations

The natural deodorant segment has grown significantly in response to consumer demand for aluminium-free and synthetic-preservative-free underarm products. It is important to distinguish two separate consumer concerns that drive this demand: a preference for ingredients of natural origin, and a specific aversion to aluminium salts. The former drives the formulation of deodorants using naturally derived antibacterial actives; the latter is a product category distinction — any product without an aluminium salt active is not an antiperspirant by definition, regardless of how effective it may be at controlling odour. Natural deodorant formulations cannot make antiperspirant claims or position themselves as sweat-reduction products without aluminium salt actives.

The most technically effective natural deodorant actives are sodium bicarbonate — which raises axillary skin surface pH from approximately 6.5–7 toward 8–9, inhibiting the acid-producing Corynebacterium species responsible for the majority of volatile fatty acid malodour — and magnesium hydroxide, which provides a similar alkalising and mild antibacterial effect with substantially lower skin irritation risk than high-concentration bicarbonate. Arrowroot powder and non-GMO tapioca starch are included as physical moisture absorbers that reduce the sensation of dampness without occluding glands. Essential oils (Australian tea tree oil, lavender, eucalyptus) provide fragrance and mild antimicrobial activity from terpenoid and phenolic components, but have low skin substantivity and short-duration antibacterial effect on the underarm microbiome. Crystal deodorants — blocks or sprays of potassium alum (KAl(SO₄)₂·12H₂O) or ammonium alum — provide aluminium ions that inhibit some odour-causing bacteria, but the aluminium species in alum does not form the gel plug required for sweat duct occlusion; they are not effective antiperspirants. The full spectrum of natural cosmetic ingredient strategy — including preservative systems, emulsifier selection, and stability assessment for natural positioning — is covered in our resource on cosmetic formulation development.

From a regulatory standpoint, natural deodorant products in the EU must comply with EU Cosmetics Regulation 1223/2009 including the restricted substance lists in Annexes II–VI, which do not exempt naturally derived ingredients from restriction — a plant-derived antimicrobial that appears in Annex II is prohibited regardless of its origin. Marketing claims such as "aluminium-free" require careful substantiation: the claim is factually accurate only if the formulation contains no aluminium compounds in any form, including alum. Claims implying equivalent efficacy to antiperspirant products without clinical substantiation risk being misleading under EU Directive 2005/29/EC on unfair commercial practices and equivalent national consumer protection legislation.