Every time you inhale vapor from an e-cigarette or vape device, you’re experiencing the carefully engineered transformation of a liquid mixture into an aerosol. But what exactly is in that liquid? Unlike traditional cigarette smoke—which contains thousands of compounds created through combustion—e-liquid formulations are remarkably straightforward, typically containing just four to six primary ingredients.
Understanding what’s inside your e-liquid, how each component functions, and how they interact is essential knowledge for anyone who vapes. Let’s break down the science, chemistry, and practical implications of each ingredient.
The Four Core Components
Modern e-liquids consist of:
- Propylene Glycol (PG) – Base carrier liquid
- Vegetable Glycerin (VG) – Base carrier liquid
- Nicotine – Active ingredient (optional)
- Flavorings – Taste compounds
Some formulations also include:
- Water – To adjust viscosity
- Ethanol or vodka – Flavor enhancer and thinner
- Sweeteners – To enhance certain flavor profiles
The ratio and quality of these ingredients dramatically affect vapor production, flavor intensity, throat hit, and overall experience. Let’s examine each component in detail.
Propylene Glycol (PG): The Flavor Carrier
What Is PG?
Propylene glycol (chemical formula: C₃H₈O₂) is a synthetic organic compound—specifically, a viscous, colorless liquid with a faintly sweet taste. Don’t confuse it with ethylene glycol (antifreeze), which is toxic; propylene glycol is generally recognized as safe (GRAS) by the FDA for consumption and has been used in food, pharmaceuticals, and cosmetics for decades.
Chemical structure: PG is a diol (contains two hydroxyl groups), making it an excellent solvent that’s miscible with water and many organic compounds.
Physical Properties
- Appearance: Clear, colorless, slightly viscous liquid
- Boiling point: 188°C (370°F)
- Viscosity: Low—flows easily, similar to vegetable oil
- Density: 1.036 g/cm³ (slightly denser than water)
- Hygroscopic: Absorbs moisture from the air
Role in E-Liquid
Flavor Carrier: PG’s excellent solvent properties make it ideal for dissolving and carrying flavor compounds. Most flavor concentrates are PG-based because it preserves flavor integrity better than VG.
Throat Hit: PG produces the sharp, slightly harsh sensation in the throat that many former smokers associate with cigarette smoking. This “throat hit” is one of PG’s most important characteristics for smokers transitioning to vaping.
Thin Consistency: PG’s low viscosity allows it to wick efficiently in coils, particularly important in smaller devices and mouth-to-lung (MTL) setups with tighter wicking channels.
Vapor Texture: PG produces thinner, less visible vapor compared to VG, but that vapor carries flavor more effectively.
The Science of Throat Hit
The physical mechanism behind PG’s throat hit isn’t entirely understood, but research suggests:
TRPV1 Receptor Activation: PG appears to mildly activate TRPV1 receptors in the throat—the same receptors triggered by capsaicin (chili peppers) and heat. This creates the sharp, prickly sensation vapers describe as throat hit.
Dehydration Effect: PG’s hygroscopic properties may temporarily dehydrate mucous membranes, contributing to the sensation. This is also why some users experience dry mouth or throat with high-PG liquids.
pH and Acidity: PG in vapor form has a slightly acidic pH, which may contribute to throat stimulation.
Common Uses Beyond Vaping
PG’s safety profile is well-established through its widespread use in:
- Food industry: Preservative, moisture retention agent, solvent for food colorings and flavors
- Pharmaceuticals: Carrier for injectable medications, oral medicines, and topical preparations
- Cosmetics: Moisture retention in lotions, soaps, and shampoos
- Fog machines: Theatrical fog and artificial smoke effects
- Air sanitization: Hospital HVAC systems (PG vapor has antimicrobial properties)
The FDA has classified PG as GRAS (Generally Recognized As Safe) for ingestion. However, inhalation safety is studied separately and has different considerations—more on this in the safety section.
PG Sensitivity and Allergies
A small percentage of people (estimated 1-2%) experience sensitivity or allergic reactions to PG:
Symptoms:
- Throat irritation or soreness
- Dry mouth and increased thirst
- Skin reactions (rashes, itching)
- Respiratory irritation
- Headaches
Solutions:
- Switch to high-VG or max-VG e-liquids
- Increase water intake
- Try 100% VG liquids (though flavor options are more limited)
- Consider nicotine salt formulations, which may be smoother
True PG allergy is rare, but sensitivity is common enough that most manufacturers offer high-VG alternatives.
Vegetable Glycerin (VG): The Cloud Producer
What Is VG?
Vegetable glycerin (also called glycerol; chemical formula: C₃H₈O₃) is a simple sugar alcohol derived from plant oils—typically palm, soy, or coconut oil. Like PG, it’s a clear, odorless liquid, but VG is noticeably thicker and sweeter.
Chemical structure: VG contains three hydroxyl groups (compared to PG’s two), making it even more hygroscopic and contributing to its higher viscosity.
Physical Properties
- Appearance: Clear, colorless, very viscous liquid
- Boiling point: 290°C (554°F)
- Viscosity: High—thick and syrupy, similar to honey
- Density: 1.261 g/cm³ (denser than PG)
- Taste: Noticeably sweet
- Hygroscopic: Strongly absorbs moisture
Role in E-Liquid
Vapor Production: VG is responsible for the dense, voluminous clouds that have become synonymous with modern vaping. Its higher boiling point and molecular structure produce significantly more visible aerosol than PG.
Smooth Inhale: VG produces a much smoother, less harsh throat sensation than PG. High-VG liquids feel gentler on the throat and lungs, making them popular with cloud chasers and those who find PG too harsh.
Sweetness: VG’s natural sweetness adds a subtle sweet undertone to e-liquids, which can enhance dessert and fruit flavors but sometimes mutes tobacco or savory profiles.
Thick Consistency: VG’s syrupy viscosity requires larger wicking ports and more porous cotton. Sub-ohm coils and direct-to-lung (DTL) devices are specifically designed to handle high-VG liquids.
The Physics of Cloud Production
Why does VG produce bigger clouds than PG?
Molecular Size: VG molecules are larger and more complex than PG molecules. When vaporized, they create more suspended particulate matter per unit of liquid.
Boiling Point: VG’s higher boiling point (290°C vs. PG’s 188°C) means it requires more energy to vaporize, but produces more aerosol density when it does.
Hygroscopic Nature: VG aggressively absorbs water vapor from your breath and the surrounding air, causing vapor droplets to grow larger and more visible as they mix with atmospheric moisture.
Surface Tension: VG’s higher surface tension creates more stable aerosol particles that remain suspended longer rather than immediately dissipating.
Common Uses Beyond Vaping
VG is even more widely used than PG:
- Food industry: Sweetener, moisture retention agent, preservative
- Pharmaceuticals: Coating for pills, suppositories, cough syrups
- Cosmetics: Primary ingredient in soaps, moisturizers, toothpaste
- Explosives: Component in nitroglycerin (though the production process is completely different from food-grade VG)
- Theatrical effects: Bubble machines, fog effects
VG is also FDA-classified as GRAS for ingestion.
VG Considerations
Coil Longevity: VG’s thickness and sweetness contribute to faster “coil gunk” buildup—the dark, sticky residue that eventually degrades coil performance. High-VG liquids typically require more frequent coil changes.
Wicking Speed: VG wicks more slowly than PG due to its viscosity. Chain vaping with high-VG liquids in devices with inadequate wicking can lead to dry hits.
Flavor Muting: VG doesn’t carry flavor compounds as effectively as PG. High-VG liquids often require more concentrated flavorings to achieve the same taste intensity.
Allergies: VG allergies are extremely rare, making it the go-to choice for those with PG sensitivity.
The PG/VG Ratio: Finding Your Balance
E-liquids are described by their PG/VG ratio, which dramatically affects the vaping experience:
High PG Ratios (70/30, 60/40, 50/50 PG/VG)
Best for:
- Mouth-to-lung (MTL) devices
- Pod systems and starter kits
- Former smokers seeking strong throat hit
- Maximum flavor intensity
- Lower wattage devices (10-20W)
Characteristics:
- Strong throat hit
- Intense, clear flavor
- Less visible vapor
- Thinner consistency wicks quickly
- Lower coil stress
- Can be harsh for sensitive users
Typical uses: Nicotine salt pods, MTL tanks, beginner devices
Balanced Ratios (50/50)
Best for:
- Versatile all-day vaping
- Moderate throat hit and vapor
- Pod mods and low-wattage sub-ohm devices
- Users who want both flavor and clouds
Characteristics:
- Moderate throat hit
- Good flavor preservation
- Reasonable vapor production
- Works in most devices
- Balanced experience
Typical uses: All-purpose liquids, adaptable to various devices
High VG Ratios (70/30, 80/20, Max VG – VG/PG)
Best for:
- Direct-to-lung (DTL) sub-ohm devices
- Cloud chasing
- Smooth, gentle inhale
- High wattage devices (40-200W+)
- Users with PG sensitivity
Characteristics:
- Minimal throat hit (very smooth)
- Massive vapor production
- Slightly muted flavor (requires more flavoring)
- Very thick—requires proper wicking
- Faster coil gunk buildup
- Sweet undertone
Typical uses: Sub-ohm tanks, drippers, cloud competitions
Max VG (90/10 or 100% VG)
Best for:
- Extreme cloud chasers
- Users with severe PG sensitivity
- High-power devices with excellent wicking
Characteristics:
- Ultra-smooth
- Maximum vapor density
- Muted flavor (requires creative formulation)
- Very thick—may not work in standard devices
- Significant coil gunk
Note: True 100% VG liquids are rare because most flavoring concentrates contain PG. “Max VG” typically means VG plus only the PG from flavorings (usually 90-95% VG).
Nicotine: The Active Ingredient
What Is Nicotine?
Nicotine (chemical formula: C₁₀H₁₄N₂) is a naturally occurring alkaloid found primarily in tobacco plants (Nicotiana tabacum), though it also appears in smaller amounts in tomatoes, potatoes, eggplants, and peppers.
Chemical nature: Nicotine is a tertiary amine, existing as a colorless to pale yellow liquid in its pure form. It’s highly soluble in water and alcohol, which is why it dissolves readily in PG/VG mixtures.
Forms of Nicotine in E-Liquids
Freebase Nicotine
This is nicotine in its pure, deprotonated form—the same form found in traditional e-liquids and created through a process similar to that used in cigarette manufacturing (adding ammonia to tobacco).
Characteristics:
- pH around 8-9 (alkaline)
- Harsh throat hit at higher concentrations
- Slower absorption into bloodstream
- Better for gradual nicotine delivery
- Available in concentrations from 0mg to 24mg/ml (though 3-12mg is most common)
Best for: Sub-ohm vaping, DTL devices, users who prefer gradual nicotine delivery
The science: At higher pH, nicotine is uncharged (freebase), making it more volatile and able to penetrate membranes more easily—but this also makes it harsher on the throat.
Nicotine Salts (Nic Salts)
Nicotine salts are created by combining freebase nicotine with an organic acid (usually benzoic acid, but sometimes citric, salicylic, or levulinic acid). This lowers the pH and creates a more stable compound.
Characteristics:
- pH around 5-6 (closer to neutral)
- Much smoother throat hit even at high concentrations
- Faster absorption—more similar to cigarettes
- Better for quick nicotine satisfaction
- Available in higher concentrations (25mg-50mg/ml common)
Best for: Pod systems, MTL devices, former heavy smokers, discreet vaping
The science: The acidified form creates a salt where nicotine ions are paired with acid anions. This lower pH makes it much less harsh, allowing for higher nicotine concentrations without the throat irritation of freebase nicotine.
Historical note: Nicotine salts are actually closer to tobacco’s natural form—tobacco leaves contain nicotine salts, which cigarette manufacturers then convert to freebase for enhanced delivery.
Nicotine Strengths Explained
Nicotine concentration is measured in milligrams per milliliter (mg/ml), sometimes shown as a percentage:
- 0mg/ml (0%): Nicotine-free
- 3mg/ml (0.3%): Very light, sub-ohm vaping
- 6mg/ml (0.6%): Light, sub-ohm or moderate MTL
- 12mg/ml (1.2%): Medium, standard MTL devices
- 18mg/ml (1.8%): Strong, heavy smokers transitioning
- 25-50mg/ml (2.5-5%): Nicotine salts for pod systems
Choosing your strength:
- Heavy smokers (pack+ per day): 12-18mg freebase or 35-50mg nic salts
- Moderate smokers (half pack per day): 6-12mg freebase or 25-35mg nic salts
- Light smokers: 3-6mg freebase or 20-25mg nic salts
- Sub-ohm vapers: 3-6mg freebase (the high vapor volume delivers more nicotine per puff)
How Nicotine Works in the Body
Absorption: When inhaled, nicotine is absorbed through the mucous membranes of the mouth, throat, and lungs into the bloodstream.
Distribution: Nicotine reaches the brain within 10-20 seconds, crossing the blood-brain barrier easily due to its lipophilic (fat-soluble) nature.
Mechanism: In the brain, nicotine binds to nicotinic acetylcholine receptors, triggering the release of dopamine and other neurotransmitters.
Effects:
- Stimulation and alertness
- Mood elevation
- Increased focus and concentration
- Appetite suppression
- Temporary increase in heart rate and blood pressure
- Relaxation paradoxically accompanies stimulation
Metabolism: Nicotine is primarily metabolized by the liver enzyme CYP2A6 into cotinine, with a half-life of about 2 hours. This means nicotine levels drop by half every 2 hours after your last puff.
Addiction mechanism: Regular nicotine use causes upregulation of nicotinic receptors—the brain creates more receptors to compensate for constant stimulation. When nicotine is absent, these extra receptors create withdrawal symptoms (irritability, cravings, difficulty concentrating).
Nicotine Safety Considerations
Toxicity: Pure nicotine is highly toxic—the lethal dose for an adult is estimated at 30-60mg (though recent studies suggest it may be higher). However, this refers to pure nicotine absorbed rapidly, which is virtually impossible through vaping.
E-liquid safety: A 30ml bottle of 18mg/ml liquid contains 540mg of nicotine—potentially dangerous if consumed entirely and rapidly (especially for children), but:
- Absorption through ingestion is slower and less complete than injection
- Vomiting typically occurs before lethal doses are absorbed
- Vaping delivers nicotine gradually over time, not all at once
Critical safety: Store e-liquids in child-proof containers, away from children and pets. Nicotine poisoning in children is a medical emergency.
During pregnancy: Nicotine crosses the placental barrier and can affect fetal development. Pregnant women should avoid nicotine in any form.
Nicotine Oxidation
Nicotine is susceptible to oxidation when exposed to air, light, and heat:
Color change: Fresh nicotine is clear or slightly yellow. Oxidized nicotine turns amber, brown, or even dark brown.
Taste change: Oxidized nicotine tastes peppery, harsh, and unpleasant.
Storage: Keep e-liquids in cool, dark places. Amber or opaque bottles reduce light exposure. Refrigeration extends shelf life significantly.
Still safe?: Oxidized nicotine is still safe to vape (just unpleasant), but it indicates the liquid is old and flavor compounds may also have degraded.
Flavorings: The Art and Science
What Are E-Liquid Flavorings?
E-liquid flavors use the same food-grade flavor concentrates used in candies, beverages, baked goods, and other consumables. These are complex mixtures of aromatic compounds designed to recreate specific taste experiences.
Types of Flavoring Compounds
Natural Flavorings
Derived from plant or animal sources through extraction, distillation, or concentration:
- Essential oils (citrus, mint)
- Fruit extracts
- Vanilla extract
- Tobacco absolutes
Advantages: Authentic, complex flavor profiles; “natural” marketing appeal
Disadvantages: Expensive; inconsistent between batches; potential allergens; some aren’t safe for inhalation despite being food-safe
Artificial Flavorings
Synthesized chemically to mimic natural flavors:
- Vanillin (artificial vanilla)
- Ethyl maltol (cotton candy sweetness)
- Diacetyl (butter/cream—controversial, discussed below)
- Various esters, aldehydes, and ketones
Advantages: Consistent, precise flavor; cost-effective; can create flavors impossible naturally
Disadvantages: “Artificial” perception; some compounds of concern for inhalation
Nature-Identical Flavorings
Chemically identical to natural compounds but synthesized in labs:
- Cinnamaldehyde (cinnamon)
- Linalool (floral, lavender)
- Limonene (citrus)
Advantages: Identical to natural versions; consistent; cost-effective
Flavor Categories
Fruit: Strawberry, watermelon, mango, apple, mixed berries—typically use combinations of esters and aldehydes
Dessert: Custards, cakes, cookies, ice cream—often use vanillin, ethyl maltol, and cream flavors
Beverage: Coffee, cola, energy drink—complex blends mimicking popular drinks
Tobacco: Attempts to recreate cigarette or pipe tobacco taste using tobacco extracts and complementary flavors
Menthol/Mint: Cooling agents like menthol, koolada, or WS-23 combined with mint flavors
Candy: Sweet, often fruit or dessert-inspired flavors with high sweetener content
The Inhalation Safety Question
This is the most important and complex aspect of flavorings:
Food-safe ≠ Inhalation-safe: Just because a flavoring is safe to eat doesn’t automatically mean it’s safe to inhale. The respiratory system is different from the digestive system.
Compounds of Concern:
Diacetyl and Acetyl Propionyl
What they are: Buttery-tasting compounds used in custards, creams, and pastries
The concern: Occupational exposure to high concentrations of diacetyl vapor has been linked to bronchiolitis obliterans (“popcorn lung”) in microwave popcorn factory workers.
The reality: While diacetyl in high concentrations is definitely concerning, the levels found in e-liquids (when present) are typically 100-1000 times lower than in cigarette smoke, and substantially lower than the occupational exposures that caused disease.
Current status: Most reputable manufacturers have eliminated diacetyl and acetyl propionyl from their formulations as a precautionary measure, even though evidence of harm at vaping concentrations is lacking.
Vitamin E Acetate
What it is: An oil-based thickening agent
The concern: This was identified as the primary cause of the 2019 EVALI (E-cigarette or Vaping product use-Associated Lung Injury) outbreak.
Critical note: Vitamin E acetate was found only in illicit THC vaping products, never in legitimate nicotine e-liquids. It should never be present in any inhalable product.
Status: Not used in legitimate e-liquids; any product containing it is illicit or dangerous.
Cinnamaldehyde (Cinnamon)
What it is: The compound that gives cinnamon its characteristic flavor and aroma
The concern: Some studies suggest it may be cytotoxic (toxic to cells) at high concentrations and can cause sensitization reactions.
Reality: Used in moderation in food products for centuries; at typical e-liquid concentrations, risk appears low, though some users report irritation from heavily cinnamon-flavored liquids.
Oils and Lipids
The concern: Inhaling oils can cause lipoid pneumonia—a serious condition where lipids accumulate in the lungs.
Prevention: Reputable e-liquid manufacturers never use oil-based flavorings. Water-soluble, alcohol-soluble, or PG-soluble flavorings only.
Watch for: “Natural” extracts that might be oil-based; any cloudy liquids; anything marketing “essential oils” for vaping.
Sweeteners
Many e-liquids include sweeteners to enhance flavor:
Sucralose (Splenda): Most common; heat-stable, very sweet, contributes to coil gunk
Ethyl maltol: Cotton-candy sweetness; also enhances fruit flavors
Stevia: Natural sweetener, less common due to aftertaste
Impact: Sweeteners significantly accelerate coil degradation, as they caramelize on the heating element. “Coil killers” are typically very sweet liquids.
Flavor Concentration
Flavorings typically comprise 5-20% of total e-liquid volume:
- Light flavoring: 5-10% (subtle, clean, longer coil life)
- Standard flavoring: 10-15% (balanced)
- Heavy flavoring: 15-20%+ (intense, may be harsh, faster coil degradation)
More flavoring isn’t always better—over-flavored liquids can taste perfumey or chemical.
Additional Ingredients
Water (H₂O)
Sometimes added in small amounts (5-10%) to:
- Thin high-VG liquids
- Enhance throat hit mildly
- Improve wicking in specific devices
- Reduce sweetness
Too much water can cause spitting and weaken flavor.
Ethanol (Alcohol)
Occasionally added (5-10%) to:
- Enhance certain flavors (particularly fruits)
- Thin viscous liquids
- Increase throat hit
- Improve flavor mixing
Can contribute to coil degradation and may be harsh for some users.
Distilled White Vinegar or Citric Acid
Rarely used, but can:
- Adjust pH
- Enhance sour or fruity notes
- Clean tongue for better flavor perception
Used only in tiny amounts (1-2%).
Quality and Purity: What to Look For
Pharmaceutical Grade vs. Food Grade
USP/Pharmaceutical grade: United States Pharmacopeia certified; highest purity standards (99.5%+ pure)
Food grade: Safe for consumption but may contain more impurities (typically 95-99% pure)
Industrial grade: Not safe for vaping; may contain significant contaminants
Reputable manufacturers use USP-grade PG and VG.
Certification and Testing
Look for e-liquids that:
- List all ingredients
- Provide third-party lab testing results
- Are manufactured in clean-room or ISO-certified facilities
- Comply with relevant regulations (TPD in Europe, FDA in US)
- Use childproof bottles
- Include clear nicotine warnings
Signs of Low Quality
Red flags include:
- No ingredient list
- Suspiciously low price
- Unknown manufacturer
- No nicotine warning labels
- Missing batch numbers or expiration dates
- Cloudy appearance (suggests oil contamination)
- Separation (liquid splitting into layers)
- Off colors or unusual viscosity
Shelf Life and Storage
Typical Shelf Life
- Unopened: 1-2 years from manufacture date
- Opened: 6-12 months if stored properly
- Nicotine-free: Longer (up to 2 years opened)
Proper Storage
Cool: 15-25°C (60-77°F); refrigeration extends life Dark: Amber bottles or storage away from light Sealed: Minimize air exposure Upright: Prevents leaks
Signs of Degradation
- Darkening color (nicotine oxidation)
- Separation of components
- Harsh, peppery taste
- Reduced flavor intensity
- Visible particles or cloudiness
Degraded liquid is generally safe but unpleasant.
DIY E-Liquid: Understanding the Risks and Rewards
Some vapers mix their own e-liquids from base ingredients. While this can be cost-effective and allows complete customization, it requires:
Safety knowledge: Nicotine handling (gloves, proper dilution) Precision: Accurate measurements (scales accurate to 0.01g) Understanding: Flavor percentages, steeping times, ratio calculations Quality ingredients: USP-grade bases, reputable flavor concentrates
Risks of DIY:
- Nicotine poisoning from mishandling concentrated nicotine
- Incorrect nicotine strength (potentially dangerous)
- Using unsafe flavorings (oils, non-food-grade compounds)
- Contamination from non-sterile equipment
If you’re interested in DIY, extensive research and proper equipment are essential.
The Bottom Line: Simple Ingredients, Complex Science
E-liquid formulation is deceptively complex—while the ingredient list is short, the interactions between components, the chemistry of vaporization, and the considerations for inhalation safety require significant expertise.
The key takeaways:
- PG and VG are the foundations—safe, widely used compounds that determine throat hit, flavor, and vapor production
- The ratio matters—choose based on your device and preferences (more PG for flavor/throat hit; more VG for smoothness/clouds)
- Nicotine comes in two main forms—freebase for gradual delivery, salts for quick satisfaction
- Flavorings are food-grade but require inhalation safety considerations—reputable manufacturers avoid known harmful compounds
- Quality matters enormously—pharmaceutical-grade ingredients from reputable manufacturers are worth the investment
- Proper storage extends life—keep liquids cool, dark, and sealed
Understanding what’s in your e-liquid empowers you to make informed choices, select appropriate products for your needs, and vape with greater confidence in what you’re inhaling. While no form of nicotine delivery is completely risk-free, knowing your e-liquid’s composition is the foundation of informed, responsible vaping.
The beauty of modern e-liquid formulation is that this knowledge is accessible—unlike combustible cigarettes with their thousands of compounds, you can understand and evaluate every ingredient in your vape, making truly informed decisions about what you choose to use.
