From Traffic to Headphones, Everyday Sound Exposure May Leave Lasting Mark on Heart

Everyday sounds from traffic, construction, conversations, music and podcasts may influence the body for much longer than the moment in which they are heard, according to the study 'Association between environmental and headphone noise and heart rate variability: observations from Apple Hearing Study cohort,' published in the nature Journal of Exposure Science & Environmental Epidemiology.

Researchers Xin Zhang, Sung Kyun Park, Lauren M. Smith and Richard L. Neitzel used real-world data from Apple Watches and iPhones to examine how environmental noise and headphone audio were associated with heart rate variability, revealing a pattern of brief initial changes followed by reductions that could continue for minutes or hours.

Why Heart Rate Variability Matters

Heart rate variability, or HRV, describes the small differences in time between consecutive heartbeats, reflecting how the autonomic nervous system continually adjusts the heart in response to movement, stress, rest and changing surroundings. A healthy heart does not beat with perfect mechanical regularity, and greater flexibility between beats is considered a sign that the body can respond effectively to physical and emotional demands. Continuously low HRV has been associated with stress, ischemic heart disease, cardiovascular problems and a higher risk of death from several causes, making it a useful indicator for studying the body's response to noise.

The researchers focused on the standard deviation of normal-to-normal heartbeat intervals, known as SDNN, which offers a broad measure of autonomic regulation. Environmental noise has already been connected with hypertension and ischemic heart disease, when exposure continues over long periods. Most earlier field studies examined cardiovascular responses within windows of about 15 minutes. This research explored whether the response might last as long as 160 minutes and whether age, body weight, stress, hearing ability or tinnitus could change a person's sensitivity.

Wearable Devices Captured Sound and Heart Patterns in Daily Life

The study drew from consenting adults enrolled in the nationwide Apple Hearing Study between November 2019 and December 2024. Participants shared environmental sound levels, headphone audio levels and HRV measurements collected through their devices, completed demographic and hearing surveys, and contributed at least 20 consecutive SDNN readings. The researchers created four analytical groups covering short-term environmental noise, long-term environmental noise, short-term headphone sound and long-term headphone sound.

Stratified sampling was used to balance participants across age, sex assigned at birth and ethnicity, producing groups ranging from 980 to 1,180 people. The environmental analyses contained more than 293,000 observations for the long-term window and over 361,000 for the short-term window, while the headphone analyses included roughly 160,000 observations in each time window. Environmental sound came from the Apple Watch Noise app, headphone levels were recorded through the iPhone Health app, and one-minute SDNN measurements were collected every two to four hours when the watch detected that the wearer was still.

Short-term models examined the 20 minutes preceding each HRV reading, and long-term models covered the previous 160 minutes using 20-minute sound averages. The researchers adjusted the calculations for physical activity and time of day because both can strongly affect HRV. Hierarchical Bayesian models were used to account for repeated measurements from each person and to recognize that two people exposed to the same sound level may have very different physiological responses.

Median sound levels differed by source and measurement window. Short-term environmental exposure averaged 52.3 decibels, rising to 62.0 decibels in the longer window, while headphone sound averaged 66.4 and 68.4 decibels. Median SDNN ranged from about 27 milliseconds in the environmental groups to almost 30 milliseconds in the headphone groups.

The Body's Response Continued After the Sound Was Heard

A 10-decibel increase in environmental noise from the quiet reference level of 40 decibels to 50 decibels was associated with a 6.8% overall reduction in SDNN during the short-term window and a 16% reduction across the long-term window. The equivalent increase in headphone sound was associated with reductions of 7.1% over 20 minutes and 7.4% over 160 minutes. At 60 decibels, long-term environmental exposure was linked with a 27.4% reduction, more than twice the 10.4% reduction estimated in the short-term model, while short- and long-term headphone exposure produced similar reductions of 14.3% and 14.6%.

Environmental noise was associated with a small rise in SDNN during the first six minutes, followed by little change around minutes seven and eight and sustained reductions from about eight to 20 minutes. Headphone sound produced a smaller increase during the first three minutes before reductions emerged after five minutes. Long-term environmental exposure showed reductions throughout the 160-minute window, while the effect associated with headphone sound was strongest earlier and moved closer to zero during the final 20-minute period.

Researchers suggested that sudden shifts in heart rate might explain the initial HRV increase, though the observational design could not reveal the biological mechanism. Sound exposure alone explained only a small share of the total variation in SDNN, with personal baseline differences accounting for roughly one-third to two-fifths of model variation. The results therefore point to noise as one influence within a much larger mix of physical activity, health, behaviour and individual biology.

Age, Tinnitus and Hearing Ability Shaped the Association

Older adults showed greater reductions than younger participants when exposed to the same environmental sound levels. At 60 decibels, estimated SDNN fell by 20.9% among people aged 18 to 24 and by 31.3% among those aged 65 or older. People with higher body mass index also tended to show larger changes in both directions, while sex was not clearly associated with the response and perceived stress produced only modest differences in the environmental models.

At 60 decibels of environmental noise, participants experiencing tinnitus a few times a year or several times a day had reductions of around 28%, compared with 25.6% among those who never reported tinnitus. Headphone users with tinnitus also showed larger reductions across several exposure levels, although the pattern was less consistent among those reporting symptoms several times a day.

Hearing ability produced one of the most striking contrasts between sound sources. People rating their hearing as excellent had larger HRV reductions during loud environmental noise, including a 35.4% decline at 70 decibels, compared with 30.5% among those reporting poor hearing. The pattern reversed during headphone listening: people with poor hearing experienced larger reductions than those with excellent hearing, possibly because understanding music, speech or podcasts requires greater mental effort when hearing is impaired.

These findings describe associations rather than proof that noise directly caused the HRV changes. Participants were Apple product users and may not represent the wider population; some older demographic groups were excluded because too few eligible participants were available, and the consumer devices were not individually calibrated. Apple Watch SDNN readings may underestimate reference measurements, environmental sound readings can be affected by clothing or water, and the study lacked detailed information on sleep, caffeine, alcohol, sound characteristics and hearing protection. Only SDNN was available, leaving other HRV measures unexplored, and uncertainty from the first modelling stage was not carried into the subgroup analysis.