# The Selective Detection of Individual Respiratory Droplets in Air

## TL;DR

A new sensor can detect and count virus-laden respiratory droplets in the air in real time, helping prevent the spread of airborne diseases.

## Contribution

A capacitive sensor that selectively detects individual respiratory droplets in real time, independent of other airborne particles.

## Key findings

- Respiratory droplet concentrations correlated with human occupancy in a nonventilated room.
- The sensor distinguishes droplets from PM by exploiting differences in dielectric constants.
- The technology enables real-time monitoring for infection prevention in public spaces.

## Abstract

Preventing the spread
of airborne diseases in crowded
indoor spaces
is a global challenge. Infected individuals release virus-laden respiratory
droplets (RDs) that can remain suspended in air and infectious for
hours. Current monitoring methods cannot distinguish these droplets
from airborne particulate matter (PM) in a real time. Here, we present
a capacitive sensor that selectively detects and counts the individual
droplets in indoor spaces, regardless the presence of PM. The device
exploits the dielectric constant (ε) of water (78.2) to differentiate
the droplets from solid PM particles (ε < 15). In a nonventilated
conference-room study, RDs concentrations (40–330 RDs/L) were
found to be correlated with human occupancy, but not with PM2.5 levels. The developed technology enables a real-time monitoring
of number concentration of RDs, which represent a potential health
risk when they carry viral or bacterial infections. The detected increase
in RD concentration can serve as a trigger for data-driven ventilation
and infection-prevention measures, providing an effective tool for
mitigating the spread of respiratory diseases in hospitals, schools
and other public spaces.