Modern WisdomThe Science Of How Smells Work - Harold McGee | Modern Wisdom Podcast 257
CHAPTERS
- 0:00 – 0:19
Smell as intimate contact: molecules briefly become “part of you”
Harold opens with a vivid explanation of why smell feels uniquely intimate: receptors physically bind to odor molecules. This frames smell as a direct, bodily encounter with the outside world rather than a distant signal like light or sound.
- •Olfactory receptors bind to molecules to detect them
- •Smelling involves literal molecular contact with the source
- •This intimacy explains instinctive aversion (e.g., holding your nose)
- •Sets the tone for smell as a deeply embodied sense
- 0:19 – 1:08
Harold’s decade-long “field guide” obsession with everyday smells
Chris asks what Harold has been researching, and Harold describes spending years cataloging smells people usually ignore. The conversation highlights how vast and surprising the olfactory world is, even in ordinary objects.
- •Harold has focused specifically on smells as phenomena in the world
- •Everyday environments contain overlooked odor signatures
- •The project resulted in a large, comprehensive book
- •Curiosity and attention are central to improving smell awareness
- 1:08 – 2:36
What a smell is: brain-made perception triggered by airborne molecules
Harold defines smell as a perception constructed in the brain but initiated by volatile molecules escaping objects and entering the nose. He contrasts smell with vision and hearing as a more direct informational link to “things themselves.”
- •Smell is generated in the brain, stimulated by external molecules
- •Volatiles travel through air and are inhaled into the nose
- •Receptors detect and report; the brain integrates context and memory
- •Smell is portrayed as our most direct contact with the material world
- 2:36 – 4:21
Where smell “happens”: why flavor feels in the mouth (retronasal smell)
They explore why we don’t experience smell as happening in the nose, even though detection occurs there. Harold explains retronasal olfaction—air moving from mouth to nose during eating—and how the brain ‘relocates’ aroma into the mouth to create flavor.
- •We perceive smells as belonging to objects, not our nose
- •Flavor is dominated by smell (aroma), detected in the nasal cavity
- •Exhaling routes mouth volatiles into the nose (retronasal pathway)
- •The brain binds smell to the site of action (the mouth)
- 4:21 – 7:25
Breathing and sniffing strategies: how sampling shapes perception
Chris asks whether breathing technique changes taste and how short sniffs differ from long inhales. Harold explains that repeated short sniffs prevent adaptation, allow receptor reset, and give the brain multiple ‘samples’ to identify an odor—mirroring animal behavior.
- •Breathing depth, distance, and frequency change odor intake
- •Sniffing is active sampling to improve identification
- •Long continuous exposure leads to adaptation and reduced attention
- •Animals sniff in extremely short bursts; humans can train attention
- 7:25 – 8:38
Smell as ‘volatile touch’: binding, intimacy, and disgust responses
Chris likens smell to touch with volatiles entering you; Harold expands the analogy by emphasizing receptor binding. They connect this intimacy to why unpleasant odors provoke immediate protective behavior and strong reactions.
- •Smell resembles touch: ‘bits of the thing’ enter the body
- •Receptor-molecule binding makes smelling physically intimate
- •Disgust/avoidance behaviors reflect perceived internal contamination
- •Highlights why smell can feel more invasive than other senses
- 8:38 – 11:01
Why smell exists at all: the oldest sense and bacteria-level chemosensing
Harold traces smell back to early life, arguing chemical sensing predates complex senses like vision. They discuss bacteria detecting molecules (e.g., glucose) and changing movement/behavior—an ancestral foundation for taste and smell.
- •Chemical sensing is evolutionarily ancient, likely earliest sense
- •Bacteria detect molecules and alter behavior accordingly
- •Smell and taste are both chemical senses with shared ancestry
- •Eyes are complex; chemosensing comes first in evolutionary history
- 11:01 – 12:28
Taste vs smell: a dozen tastes vs ~400 receptors and combinatorial richness
Chris wonders whether smell has ‘basic categories’ like taste; Harold explains it doesn’t. Smell relies on hundreds of receptors combined in countless patterns, producing enormous discriminatory potential compared to the limited taste palette.
- •Taste has a small set of primary sensations (sweet/sour/salty/bitter/umami, etc.)
- •Smell uses ~400 receptor types in combinatorial coding
- •This enables detection/representation of vast numbers of odorants
- •Smell provides highly specific information about the world
- 12:28 – 15:48
Losing smell (COVID and beyond): food pleasure, texture, and ‘ungrounded’ flavor
They address why people undervalue smell until it’s gone, connecting to COVID-related smell/taste disruption and Harold’s own temporary loss. Harold describes how eating becomes joyless, and how taste-only or smell-only experiences can feel disconnected and unsatisfying.
- •COVID spotlighted anosmia/parosmia and sensory disruption
- •Harold lost smell while writing, changing eating from pleasure to mere fueling
- •Without smell, people compensate with texture and trigeminal ‘heat’
- •Without taste, smells can feel like ‘perfume’—not grounded in eating
- 15:48 – 19:00
Smell in entertainment: Smell-O-Vision, scratch-and-sniff, and why delivery is hard
Chris asks about smell technology; Harold recounts historical attempts like Smell-O-Vision and John Waters’ scratch-and-sniff cinema. They explain the core logistical problem: odors linger, are hard to control in large venues, and require purpose-built systems (like theme park rides).
- •Scratch-and-sniff worked surprisingly well and lasts decades
- •Digital ‘smell boxes’ are still limited in realism/effectiveness
- •Theme parks can integrate controlled scent bursts (grass, ocean spray)
- •Odor persistence and ventilation make rapid scene-to-scene switching difficult
- 19:00 – 21:08
Smells and physiology: limited universal effects and strong cultural associations
Chris probes sleep sprays and ‘alertness’ scents; Harold notes evidence for broad relaxing vs stimulating effects but emphasizes culture and association. He explains smells as multi-molecule bouquets (like musical chords), complicating simple cause-effect claims.
- •Studies suggest some scents correlate with relaxation/stimulation
- •Effects appear culture-dependent and association-driven
- •Odors are bouquets/chords, not single ‘lemon molecule’ experiences
- •Lavender vs citrus differences are intriguing despite shared components
- 21:08 – 23:40
How we interpret smells: learning, toddler neutrality, and evolving disgust
They dig into why smells feel meaningful—how memory and narrative attach to odors. Harold notes toddlers can be more neutral toward odors than adults, raising questions about developmental windows of flexibility and how aversions are shaped.
- •Smell meaning is heavily shaped by experience and memory
- •Evolution helps explain attraction/aversion, but development complicates it
- •Toddlers show greater odor neutrality and less automatic revulsion
- •Why this flexibility window exists remains an open question
- 23:40 – 26:27
Rare and crafted aromas: Japanese ‘incense listening’ and agarwood/oud
Chris asks about unique smells; Harold describes entering the perfume/incense world and learning Japan’s focused practice of ‘incense listening.’ He highlights agarwood/oud—resin formed when trees respond to fungal infection—as a layered, difficult-to-describe but compelling aroma.
- •Perfume/incense opened new domains beyond food aromas
- •Incense listening emphasizes focused attention and comparison
- •Agarwood/oud is prized for depth and evolving layers
- •Its aroma arises from a tree’s defensive resin response to fungus
- 26:27 – 32:39
Animal-derived smells: ambergris, excremental ‘bouquets,’ and the microbiome signal
They move into animal smells used in perfumery, especially ambergris from sperm whales, often found washed ashore. Harold then explores why excrement smells the way it does and how it may reflect gut microbiome activity, diet, and health signals—plus striking animal behaviors that invert human disgust.
- •Animal materials can be highly valued in perfumery
- •Ambergris: beach-found whale product with unexpectedly floral notes
- •Fecal odor reflects microbial byproducts and can indicate gut ecology
- •Some animals (e.g., gorillas) may smell/consume excrement, challenging ‘objective’ disgust
- 32:39 – 35:25
Plants and the smell of nature: pleasant molecules as defensive chemical warfare
Chris asks about plant smells; Harold contrasts animal odors (often breakdown products) with plant odors (often synthesized). Many pleasant plant volatiles are complex defensive compounds meant to repel herbivores or signal damage, even though humans interpret them as delicious or fresh.
- •Animal smells often derive from tissue breakdown and microbial metabolism
- •Plant smells are frequently synthesized for specific biological purposes
- •Complex molecules are more often perceived as pleasant than decay signals
- •Herbs/spices/green smells can be plant defense—chemical warfare and signaling
- 35:25 – 38:27
Smell vs sight: why odor is emotional, episodic, and ‘hardwired’ to memory
Chris contrasts smell’s emotional weight with vision’s dominance in sensory cells; Harold explains olfaction’s more direct pathway to emotional brain centers. He also argues smell is episodic and easily adapted away, making individual odor moments more anchorable to memory than continuous visual input.
- •Olfaction routes more directly to emotional centers than other senses
- •This may reflect smell’s evolutionary antiquity
- •Vision is continuous; smell is easier to tune out via adaptation
- •Episodic odor events become powerful memory anchors
- 38:27 – 42:20
Earthy odors decoded: geosmin, petrichor (and ‘Gaiaichor’), and place-smell chemistry
They discuss the smell of land and water: fresh soil’s signature molecule geosmin and how its biological purpose is still uncertain. Harold reframes petrichor as a release of accumulated local airborne compounds—suggesting it’s more the smell of a place than of stone itself, and varies by location and time.
- •Geosmin dominates ‘fresh dug soil’ smell and is microbially produced
- •Science still debates why geosmin is made (signal vs other function)
- •‘Petrichor’ largely reflects released accumulated volatiles on wetting surfaces
- •The smell after rain should vary dramatically by environment; ‘Gaiaichor’ as alternative concept
- 42:20 – 44:38
Humans vs bloodhounds: sensitivity, context, and the power of interpretation
Chris asks about bloodhound-level smelling; Harold notes humans can detect some compounds at parts-per-billion (e.g., geosmin). He argues animal-vs-human comparisons ignore context and cognition: humans may have fewer receptors but far greater interpretive processing, as shown by expert tasters’ discrimination abilities.
- •Humans can match extreme sensitivity for certain molecules
- •Bloodhound superiority is context-bound (ground-level tracking lifestyle)
- •Human expertise (e.g., wine tasting) shows powerful odor interpretation
- •Smell is not just detection threshold; it’s meaning-making and inference
- 44:38 – 48:23
Fire and deep time: smoke as an ancient signal and imagining prehistoric Earth odors
They explore industrial/burning smells and why humans are biologically familiar with fire: it’s been present on Earth for hundreds of millions of years and crucial to humans for ~a million years. They then imagine prehistoric atmospheres—before oxygen, a sulfurous world—and how photosynthesis reshaped the planet’s odor profile.
- •Fire predates humans; natural ignition made smoke a recurring ecological signal
- •Humans evolved strong sensitivity to smoke for survival and later dependence
- •Pre-oxygen Earth likely smelled strongly sulfurous (H₂S, thiols, etc.)
- •Photosynthesis oxygenated the atmosphere, reducing pervasive sulfur odors
- 48:23 – 53:26
Practical takeaways and what’s next: paying attention, tea chemistry, and future scent science
Chris asks how smell research changed Harold’s food life; Harold says the main shift is deeper attention and curiosity, plus exploring more aromatic diversity (including growing tea). They close on advances in volatile detection and mapping brain circuitry for perception, followed by book and website plugs.
- •Key change: noticing and diagnosing aromas rather than dismissing them
- •Greater experimentation with diverse ingredients; tea leaf aroma transformations
- •New tools can capture and analyze trace volatiles in situ (e.g., flowers in jungles)
- •Neuroscience progress on olfactory circuitry and conscious perception; links to Harold’s site/book
