Types and Grades of Hearing Loss — A Clinical Classification Guide
Flocolor Professional Hearing Health Guide
From hard to hear to total hearing loss: you may be at a certain stage
Have you ever experienced this — at a lively family dinner, you can hear everyone talking, but you just can't keep up with the conversation; or when a friend calls, if they speak slightly fast, you have to ask them to repeat themselves over and over. Many people think: "I can still hear, I just occasionally have trouble understanding." But in reality, hearing loss is often a gradual, staged process — from mild inconvenience to severe social isolation, with clear boundaries in between.
In the previous chapter, How Do We Hear?, we learned about the complete auditory pathway and the basic concepts of hearing loss. This chapter goes deeper: How many types of hearing loss are there? How do you determine which grade you fall into? And how do you read the audiogram — the chart that doctors use?
I. Hearing Loss Grading: How Severe Is Your Hearing Impairment?
The degree of hearing loss is typically measured in "Hearing Level" (dB HL). The higher the dB HL value, the louder a sound must be for it to be heard — meaning the hearing loss is more severe.
However, there is a fact that is easily overlooked: different authoritative organizations do not use exactly the same grading standards. For the general public, understanding this difference is crucial — otherwise you might receive different conclusions from different clinics.
WHO 2021 Standard (Latest Version)
In 2021, the World Health Organization (WHO) published the World Report on Hearing, updating its hearing loss grading system. This is currently the most widely adopted standard in global public health.
| Severity | PTA (Better Ear) | Everyday Impact |
|---|---|---|
| Normal | ≤20 dB HL | No noticeable difficulty |
| Mild | 21–35 dB HL ¹ | Slight difficulty, especially in noisy environments |
| Moderate | 36–50 dB HL | Moderate difficulty, everyday conversation becomes challenging |
| Moderately Severe | 51–65 dB HL | Noticeable difficulty, hearing aids recommended |
| Severe | 66–80 dB HL | Severe difficulty, hearing aids or cochlear implants needed |
| Profound | ≥81 dB HL | Nearly impossible to communicate by hearing alone |
Source: WHO World Report on Hearing, 2021
¹ The upper boundary of "Mild" hearing loss is cited as 34 dB HL in some sources and 35 dB HL in others. Readers are advised to consult the WHO World Report on Hearing (2021, Table 3.1) for the definitive figure. All grades are based on the pure-tone average threshold (0.5, 1, 2, 4 kHz) in the better-hearing ear.
Key change: Compared to the previous version, WHO 2021 lowered the "normal" upper threshold from 25 dB HL to 20 dB HL. This means that if someone's hearing falls between 21–25 dB HL, they were still considered "normal" under the old standard, but are now classified as having "mild hearing loss."
WHO 2008 Standard (Old Version)
The WHO 2008 standard is still cited by some countries and academic publications. Its most notable feature is setting the "normal" range at ≤25 dB HL.
| Severity | PTA (Better Ear) |
|---|---|
| Normal | ≤25 dB |
| Mild | 26–40 dB |
| Moderate | 41–60 dB |
| Severe | 61–80 dB |
| Profound | ≥81 dB |
Data source：WHO Grades of Hearing Impairment, 2008
Key differences from WHO 2021: WHO 2008 sets the "normal" upper limit at 25 dB, does not include the "moderately severe" sub-grade (using 21–35 / 36–50 / 51–65 instead), and the profound threshold remains at 81 dB (same as the 2021 version).
American Clinical Standards (AAO-HNS / ASHA)
The American Academy of Otolaryngology–Head and Neck Surgery (AAO-HNS) and the American Speech-Language-Hearing Association (ASHA) use a grading standard that is the most commonly applied in U.S. clinical practice. Its most notable feature is finer granularity — with six grades in total, where the "moderately severe" (56–70 dB HL) and "severe" (71–90 dB HL) ranges differ significantly from WHO standards.
| Severity | PTA (Better Ear) | Everyday Impact |
|---|---|---|
| Normal | 0–25 dB HL | No noticeable difficulty |
| Mild | 26–40 dB HL | Difficulty hearing whispers and distant speech |
| Moderate | 41–55 dB HL | Everyday conversation requires raised voices |
| Moderately Severe | 56–70 dB HL | Hearing aids needed for daily communication |
| Severe | 71–90 dB HL | Must shout to be heard |
| Profound | ≥91 dB HL | Nearly impossible to communicate by hearing alone |
Data source: AAO-HNS / ASHA grading standards
Core Differences Among Three Standards
The table below summarizes the most critical differences for quick reference.
| Comparison dimension | WHO 2021 | WHO 2008 | AAO-HNS / ASHA |
|---|---|---|---|
| Upper limit of normal | 20 dB HL | 25 dB | 25 dB HL |
| Total grades | 6 grades | 5 grades | 6 grades |
| Mild range | 21–35 dB HL | 26–40 dB | 26–40 dB HL |
| Moderate range | 36–50 dB HL | 41–60 dB | 41–55 dB HL |
| Moderately severe range | 51–65 dB HL | No tier | 56–70 dB HL |
| Severe range | 66–80 dB HL | 61–80 dB | 71–90 dB HL |
| Start of profound level | 81 dB HL | 81 dB | 91 dB HL |
| Applicable scenarios | Global public health statistics | Partial academic literature | American clinical practice |
Practical tip: When reviewing an audiogram report, check whether the grading standard is noted at the bottom. If not, ask your audiologist. At Flocolor, we clearly indicate the grading standard used in every report.
II. Types of Hearing Loss: Where Does the Problem Lie?
In the previous chapter, we introduced the basic concepts of three types of hearing loss. This chapter, using the audiogram as a core diagnostic tool, will help readers understand how each type appears on the chart.
Conductive Hearing Loss
Core feature: There is an obstruction along the sound conduction pathway from the outer ear to the middle ear, but inner ear function remains largely normal.
On the audiogram:
- Air conduction (red O / blue X, measured via headphones) shows decreased thresholds — because sound waves are blocked
- Bone conduction (red < / blue >, measured via bone oscillator) remains normal or near-normal — because inner ear function is intact
- Air-Bone Gap > 10 dB HL — the core diagnostic hallmark of conductive hearing loss
Common causes: Earwax (cerumen) impaction blocking the ear canal, tympanic membrane perforation, middle ear effusion (e.g., otitis media with effusion), otosclerosis (stapes fixation).
Good news: Conductive hearing loss is treatable in many cases — through earwax removal, medication for middle ear infections, or surgical repair.
Sensorineural Hearing Loss, SNHL
Core feature: The hair cells of the inner ear or the auditory nerve are damaged, unable to convert mechanical sound vibrations into electrical signals that the brain can recognize.
On the audiogram:
- Air and bone conduction thresholds nearly overlap — because the problem is not in the conduction pathway but in the inner ear itself
- No significant air-bone gap (≤ 10 dB HL) — the key distinction from conductive hearing loss
Common causes: Presbycusis (age-related hearing loss), noise exposure, ototoxic medications (such as certain antibiotics and chemotherapy drugs), genetic factors.
Important note: Sensorineural hearing loss is typically irreversible, because human hair cells do not regenerate once damaged. Early detection and timely intervention (such as hearing aids) are key to minimizing its impact on daily life.
The audiogram curve of SNHL takes various shapes, and different patterns often suggest different underlying causes:
| Curve Pattern | Description | Common Association |
|---|---|---|
| Sloping | Normal low frequencies, sharp decline at high frequencies | Most common; "can hear but can't understand" |
| Flat | Similar thresholds across all frequencies | Hearing aids typically work best |
| Reverse Slope | Good high frequencies, poor low frequencies | Relatively rare |
| Cookie-Bite | Poor mid-frequencies, better at both ends | Often associated with genetic factors |
| Notch at 4 kHz | Isolated dip at 4000 Hz | Hallmark sign of noise-induced hearing loss |
Mixed Hearing Loss
Core feature: Both conductive and sensorineural problems coexist — the conduction pathway in the outer/middle ear is blocked, AND the hair cells in the inner ear are also damaged.
On the audiogram:
- Air conduction shows significant decline — both the conduction pathway and inner ear are affected
- Bone conduction also shows some decline — the inner ear itself is damaged
- Air-bone gap is present (> 10 dB HL) — conductive component is still identifiable
- However, bone conduction thresholds are not within the normal range — indicating a concurrent sensorineural component
Common causes: Chronic otitis media (long-standing conductive issue) combined with presbycusis (sensorineural degeneration), or residual sensorineural damage following otosclerosis surgery.
III. Noise-Induced Hearing Loss, NIHL
Among all types of sensorineural hearing loss, there is one particularly special and highly preventable category — noise-induced hearing loss (NIHL). It is closely related to noise exposure in our daily lives.
What is NIHL?
NIHL refers to sensorineural hearing loss caused by damage to inner ear hair cells due to prolonged or single-episode high-intensity noise exposure. Its audiogram has a very distinctive "fingerprint" — the 4 kHz Notch.
Audiogram characteristics of NIHL
The audiogram of NIHL shows a distinctive pattern:
- Low frequency (250–1000 Hz): Normal or mild decline
- 2000 Hz：Hearing threshold starts to drop
- 4000 Hz：Sharp decline forming a distinct V-shaped notch, the most diagnostic feature of NIHL
- 8000 Hz：Partial threshold recovery compared with 4000 Hz
Specifically, when the threshold at 4000 Hz (or within the 3000–6000 Hz range) is at least 10 dB HL worse than adjacent frequencies, a notch is identified, strongly suggesting NIHL.
NIHL vs Presbycusis: How to Distinguish？
Both NIHL and presbycusis present as sensorineural hearing loss, but their audiogram patterns are distinctly different:
| Comparison dimension | NIHL | Presbycusis |
|---|---|---|
| Audiogram morphology | 4kHz notch with recovery at 8kHz | Smooth high-frequency decline without notch |
| Decline pattern | Sharp drop followed by rebound | Gradual and steady descent from high to low frequency |
| Performance at 8 kHz | Relative recovery, better than 4kHz | Continuous decline or flat threshold |
| Symmetry | Generally bilateral symmetry | Generally bilateral symmetry |
| Primary causes | Noise exposure (occupational/recreational) | Age-related hair cell degeneration |
Practical tip: If you work long-term in high-noise environments (e.g., construction sites, factory floors), annual hearing screenings are recommended, with particular attention to whether a notch is developing at 4000 Hz.
In the United States, the Occupational Safety and Health Administration (OSHA) defines a Standard Threshold Shift (STS) as an average threshold shift of ≥10 dB at 2000, 3000, and 4000 Hz compared to the baseline. Once an STS is reached, employers must take corrective action.
IV. Hearing Loss in Special Populations
Presbycusis
Presbycusis is the most common type of sensorineural hearing loss and one of the most prevalent chronic conditions in humans. Its essence is an age-related degenerative process of the inner ear involving multiple factors: hair cell degeneration, basilar membrane stiffening, vascular degeneration, and metabolic decline.
Core features:
- Bilateral symmetry: Both ears show approximately equal hearing loss
- High frequencies affected first: Initially shows decreased perception of high-frequency sounds (e.g., bird calls, phone ringtones)
- Speech understanding difficulty: Since consonants are concentrated in higher frequencies, high-frequency loss directly causes "can hear but can't understand"
- Audiogram pattern: Typical high-frequency sloping pattern, no notch
"Despite its high prevalence, ARHL remains underdiagnosed and undertreated, partly due to its gradual onset, stigma, and lack of standardized screening and management protocols."
Key statistic: According to WHO, approximately one-third of people over 65 have disabling hearing loss. For presbycusis, hearing aids are the first-line intervention — the earlier they are fitted, the better the outcome.
Congenital hearing loss in children
Childhood hearing loss differs from adult hearing loss because it directly affects language development, learning ability, and social growth — making early detection and early intervention essential.
Etiology:
- Genetic factors: Approximately 50%, with GJB2 gene mutations being the most common cause of non-syndromic hereditary hearing loss
- Non-genetic factors: Cytomegalovirus (CMV) infection, congenital rubella syndrome, prematurity, hyperbilirubinemia, ototoxic medication exposure
Special standard for children: WHO sets the threshold for "disabling hearing loss" in children at ≥31 dB in the better ear (lower than the adult threshold of 40 dB), because even mild-to-moderate hearing loss can severely affect language acquisition in children.
"1-3-6" Golden Principle
The Joint Committee on Infant Hearing (JCIH) recommends the "1-3-6" principle for infant hearing screening, which is globally recognized as the standard for early intervention:
| Timeline | Action |
|---|---|
| By 1 month | Complete hearing screening (OAE or AABR) |
| By 3 months | Complete comprehensive audiological diagnostic evaluation |
| By 6 months | Initiate early intervention services |
OAE =Otoacoustic Emissions；AABR = Automated Auditory Brainstem Response
V. How to Read an Audiogram: Beginner's Guide
The audiogram is the most fundamental tool in hearing assessment. Understanding it gives you the basic ability to "read" your own hearing status. Below is a beginner's guide for non-professionals.
Basic structure of audiogram
A standard audiogram consists of two axes and a set of symbols:
- Horizontal axis (X-axis) — Frequency (Hz): Left to right represents pitch from low to high, typically ranging from 250 Hz → 8000 Hz. The primary speech frequencies fall between 250–4000 Hz.
- Vertical axis (Y-axis) — Hearing Level (dB HL): Numbers increase from top to bottom (from -10 to 120), representing the degree of hearing loss. Higher up = better hearing; lower down = worse hearing.
Core Symbol Cheat Sheet
| Symbol | Color | Meaning |
|---|---|---|
| O | Red | Right ear, air conduction |
| X | Blue | Left ear, air conduction |
| < | Red | Right ear, bone conduction |
| > | Blue | Left ear, bone conduction |
Air Conduction：Sounds are delivered via earphones, testing the entire auditory pathway including outer, middle and inner ear.
Bone Conduction：Sound is transmitted through a bone vibrator placed on the mastoid behind the ear to directly stimulate the inner ear, bypassing the outer and middle ear. It helps determine whether conductive hearing loss is present.
Quick Judgment Mnemonic
Connect all symbols into a line——
- Circle and cross nearly overlap, angle symbols align with them → Sensorineural hearing loss
- Circle and cross obviously lower than angle symbols → Conductive hearing loss
- Circle and cross lower than angle symbols, while angle symbols are also out of normal range → Mixed hearing loss
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