Episode Summary
Executive Summary: The episode explores how COVID-19 has spotlighted the science of smell, from anosmia and parosmia to recovery and training. Dr. Pamela Dalton explains that smell loss is common, often underrecognized, and can affect mood, memory, food enjoyment, and orientation. The discussion also covers smell-based COVID screening, disease detection, and how olfaction links to brain function and emotion.
Main Topics: COVID-19 and smell loss: Dr. Dalton explains that COVID has caused an unusually high prevalence of smell loss compared with prior viral illnesses, including complete loss (anosmia), reduced sensitivity (hyposmia), and distorted smell (parosmia). How smell loss affects daily life and mental health: Losing smell can make people feel disconnected, disoriented, and less emotionally connected to loved ones, while also reducing enjoyment of food and altering routine awareness of environment. Measuring smell and developing rapid screening: The conversation details psychophysical odor testing and Monell's Sentinel smell test, a rapid three-patch card and smartphone survey intended to identify possible COVID cases more quickly than symptom or temperature screening. Recovery and olfactory training: Most people recover smell after COVID, but some experience longer-lasting effects. Dalton describes olfactory training with repeated daily exposure to odors as a strategy to stimulate regeneration and improve function. Smell as a diagnostic tool for disease: The episode reviews evidence that diseases can emit characteristic odors and discusses dog-assisted detection, including research on cancer and infection, with hopes of building electronic noses or sensors. Smell, memory, and emotion: Dalton explains that smell is tightly connected to the limbic system and hippocampus, which helps explain why odors can trigger vivid, emotional memories. The world's worst smell and cultural learning: Dalton discusses research on universally repulsive odors, concluding that a mix of decomposition and human-waste odors ('stench soup') was broadly aversive, while preferences and aversions are partly learned over development.
Key Arguments: COVID-19 made smell loss highly visible because it occurs frequently, even in otherwise mild cases, and can appear before fever or other obvious symptoms. Self-report underestimates smell dysfunction; objective testing finds more anosmia and hyposmia than people notice themselves. Parosmia may be part of the recovery process, when regenerating olfactory pathways produce distorted smell perceptions. Smell loss has real emotional and practical consequences: reduced pleasure in food, disorientation, and weakened social/emotional recognition. Olfactory training may help restart recovery by repeatedly stimulating the smell system with a small set of odors each day. COVID may disrupt supporting cells that help olfactory receptors regenerate, rather than permanently destroying receptors in all cases. Rapid smell screening could complement, not replace, diagnostic tests by identifying potentially infectious people who lack other symptoms. Disease odor research is promising because body-emitted volatiles may reveal infection, inflammation, or cancer before traditional diagnosis. Smell and memory are linked because of the olfactory system's anatomical proximity to emotion- and memory-related brain regions. There are no clear innate smell aversions like the hardwired bitter taste response; many smell preferences and aversions are learned across development and culture.
Data Points: Prevalence of willingness to give up smell: more than half - A national poll about 10 years ago found over half of young people would give up smell before their laptop or cell phone. Recovery timeline after COVID: 1-2 weeks - Many people recover smell within a week or two after initial symptoms or positive diagnosis. Recovery timeline after COVID: 2-3 months - Some patients recover more slowly, with smell fluctuating for months. Long-lasting smell loss after COVID: about 10% - Dalton estimates about 10% of COVID patients who lose smell have persistence beyond the usual recovery period. Long-term post-viral recovery window: up to 2-3 years - Even some post-viral olfactory disorders may still recover years later. Daily training frequency: a couple of times a day - Olfactory training involves smelling selected odorants repeatedly every day. Number of odorants in training: 4 or 5 - The suggested olfactory training set uses four or five different odors. Rapid test card size: 5 by 7 inches - The proposed Sentinel smell test is described as a single card roughly five by seven inches. Number of test patches: 3 - The Sentinel card contains three patches, only one of which contains an odor. Time for first test: about 2 minutes - The smell test can be completed in roughly two minutes the first time. Time for repeat use: less than 1 minute - Subsequent tests should take under a minute. Number of receptors: about 400 - Dalton says humans have about 400 receptor types involved in smell. Ability to discriminate smells: hundreds of thousands - Humans can distinguish very large numbers of odors through receptor combinations. Child developmental inflection: around age 8 - Earlier Monell work suggested children begin matching parents' smell preferences and aversions around age eight.
Pivotal Quotes: "It is the last sensory system to give up its secrets to science." — Dr. Pamela Dalton: Dalton describes how difficult it has been for scientists to understand smell compared with vision and hearing. "It’s sort of the backdrop of everywhere we go and everyone we interact with." — Dr. Pamela Dalton: She explains why people often underestimate smell until it is lost. "We ended up finding a mixture of scents that comprised both organic decomposition ... and a little bit of human waste ... we came up with something that we called stench soup." — Dr. Pamela Dalton: She describes how her team identified a universally repulsive odor mixture in the 'world's worst smell' study.
Implications: Smell is not a minor sense; it affects health, safety, memory, and quality of life. Better smell testing could improve COVID screening and broader disease detection, while rehabilitation may help many patients recover function over time.