Respiratory infections, ventilation and outbreaks

Respiratory infections cause more lost work than any other kind of infection. Colds, influenza, COVID-19, respiratory syncytial virus (RSV), pertussis (whooping cough), and less commonly tuberculosis and measles, all spread mainly through the air people breathe out. Lesson 2 explained that people exhale a range of particle sizes and that small aerosols can build up indoors. This lesson turns that understanding into practical controls: keeping sick people away from others, ventilation and filtration, masks and respirators, respiratory etiquette, and detecting and managing outbreaks.

Layers of protection

No single measure stops respiratory spread. Workplaces that do well use several layers, each of which removes part of the risk:

• staying home when sick, with policies and sick leave that make it possible (lesson 7);
• vaccination for the infections where vaccines are available (lesson 7);
• ventilation, filtration and, where appropriate, air cleaning;
• reducing crowding and the duration of close contact, for example by staggering breaks and using larger rooms;
• respiratory etiquette: covering coughs and sneezes, using tissues and cleaning hands afterward;
• masks for source control and personal protection, and respirators where the risk is higher (see HSE-240);
• screening for symptoms in higher-risk settings, such as health care and long-term care;
• early detection and response to clusters of illness.

Ventilation basics

Ventilation dilutes and removes airborne contaminants by replacing room air with outdoor air or with recirculated air that has passed through a filter. Two common measures describe it. Air changes per hour (ACH) is the volume of air supplied to a room in an hour divided by the room's volume. Outdoor air per person, usually in litres per second per person, describes how much fresh air each occupant receives; building codes set minimum rates based on occupancy and use.

When a source stops emitting, for example when an infectious patient leaves a room, the concentration of airborne particles falls exponentially. The CDC's guidelines for environmental infection control in health care give a table of the time needed to remove 99 per cent or 99.9 per cent of airborne contaminants at different air change rates, assuming perfect mixing. Real rooms mix imperfectly, so actual times are longer.

import math
# Minutes to remove 99% and 99.9% of airborne contaminants (well-mixed room)
print("ACH   99%   99.9%")
for ach in (2, 4, 6, 12, 15):
    t99 = -math.log(1 - 0.99) / ach * 60
    t999 = -math.log(1 - 0.999) / ach * 60
    print(f"{ach:>3}  {t99:4.0f}  {t999:5.0f}")

ACH   99%   99.9%
  2   138    207
  4    69    104
  6    46     69
 12    23     35
 15    18     28

The results match the CDC table: at 6 air changes per hour it takes about 46 minutes to remove 99 per cent of airborne contaminants and 69 minutes for 99.9 per cent; at 12 ACH, half that. This is why health care facilities keep a room closed for a set time after a patient with an airborne infection such as measles or tuberculosis leaves, and why that time depends on the room's ventilation rate.

Practical ventilation improvements

Most buildings are ventilated by heating, ventilation and air-conditioning (HVAC) systems designed to meet building codes and, for offices and schools, standards such as ASHRAE 62.1. Health care facilities in Canada follow CSA Z317.2:24, which sets minimum air change rates, pressure relationships and filtration for different rooms, including airborne infection isolation rooms kept at negative pressure. Practical steps in any building include:

• having a qualified person check that the system delivers the outdoor air it was designed for, that dampers work and that filters fit without gaps;
• upgrading filters to MERV 13 or higher where the system can handle the extra resistance;
• running ventilation during occupied hours and for a period before and after;
• using portable HEPA air cleaners sized for the room in spaces with poor ventilation, such as small meeting rooms or clinic rooms;
• opening windows where safe and practical;
• using carbon dioxide (CO₂) monitors as a rough indicator of whether ventilation is keeping up with occupancy.

Carbon dioxide is exhaled by everyone, so its concentration rises in a crowded, poorly ventilated room. Outdoor air is around 420 ppm. Indoor levels that stay below about 800 to 1,000 ppm in an occupied room usually indicate reasonable outdoor air supply for the number of people present. CO₂ does not measure germs and is not useful for spaces where air cleaners remove particles without bringing in outdoor air, but it is a cheap way to find rooms that need attention.

Portable HEPA air cleaners are rated by clean air delivery rate (CADR). Dividing the CADR by the room volume gives the additional effective air changes the unit provides.

# Equivalent air changes from a portable HEPA unit (illustrative)
room_l, room_w, room_h = 6.0, 5.0, 2.7          # m
volume = room_l * room_w * room_h               # m3
cadr_cfm = 300                                  # clean air delivery rate, cubic feet per minute
cadr_m3h = cadr_cfm * 1.699                     # 1 cfm = 1.699 m3/h
existing_ach = 2.0
added = cadr_m3h / volume
print(f"room volume {volume:.0f} m3")
print(f"HEPA unit adds about {added:.1f} air changes per hour")
print(f"total about {existing_ach + added:.1f} ACH")

room volume 81 m3
HEPA unit adds about 6.3 air changes per hour
total about 8.3 ACH

Masks, respirators and respiratory etiquette

Masks serve two purposes. As source control, a mask worn by an infected person reduces the particles they release into the air. As personal protection, a mask or respirator reduces what the wearer breathes in. Medical (surgical or procedure) masks protect against splashes and larger particles and offer some protection against smaller ones, but air leaks around their edges. Respirators such as N95 filtering facepieces are designed to fit tightly and filter at least 95 per cent of small particles, and their protection depends on fit testing and a good seal each time they are worn. HSE-240 covers respirator selection, fit testing and the law.

Health care settings decide whether to use medical masks or respirators for respiratory infections through a point-of-care risk assessment and the precautions required for each disease, as lesson 10 explains. Outside health care, masks are most useful during community outbreaks, for workers with symptoms who must be around others, for workers who are at higher risk of severe illness, and when public health recommends them.

Respiratory etiquette is simple: cover coughs and sneezes with a tissue or the inside of your elbow, put used tissues in the garbage, clean your hands afterward, and keep distance from others when you have symptoms. Workplaces can support it by providing tissues, no-touch waste bins and hand rub at entrances and in common areas.

Recognizing an outbreak

An outbreak is more cases of an illness than would normally be expected in a place and time. In long-term care, public health agencies set case definitions; a respiratory outbreak might be declared with two or more cases of acute respiratory infection with onset within a set number of days in a specific area. Workplaces outside health care rarely have formal definitions, but a cluster of absences with similar symptoms in one team, shift or location is a signal to act.

Simple measures describe an outbreak. The attack rate is the number of people who became ill divided by the number at risk. The secondary attack rate is the proportion of contacts of a case who became ill afterwards, which shows how readily the infection spreads from person to person. An epidemic curve is a histogram of new cases by date of symptom onset; its shape suggests whether the outbreak came from a single exposure (a sharp peak) or is spreading from person to person (successive waves).

from collections import Counter
# Line list of an invented respiratory cluster in a contact-handling office: (person, team, onset day)
cases = [("A", "north", 1), ("B", "north", 3), ("C", "north", 3), ("D", "south", 4),
         ("E", "north", 4), ("F", "north", 5), ("G", "south", 6), ("H", "north", 6),
         ("I", "south", 7), ("J", "north", 8)]
staff = {"north": 40, "south": 35}
for team, n in staff.items():
    ill = sum(1 for c in cases if c[1] == team)
    print(f"{team}: {ill}/{n} ill, attack rate {ill / n:.0%}")
curve = Counter(day for _, _, day in cases)
print("epidemic curve:")
for day in range(1, 9):
    print(f"day {day}: {'#' * curve.get(day, 0)}")
# Secondary attack rate among 12 close desk-mates of the first case
print(f"secondary attack rate: {3 / 12:.0%} of close contacts of case A")

north: 7/40 ill, attack rate 18%
south: 3/35 ill, attack rate 9%
epidemic curve:
day 1: #
day 2: 
day 3: ##
day 4: ##
day 5: #
day 6: ##
day 7: #
day 8: #
secondary attack rate: 25% of close contacts of case A

Managing an outbreak

In long-term care, hospitals, child care and schools, report suspected outbreaks to the local public health unit as required, and follow its direction. Typical outbreak measures include:

• confirming the cause through testing where appropriate;
• excluding ill staff until they are no longer infectious, following public health guidance;
• increasing cleaning and disinfection of high-touch surfaces;
• cohorting: assigning staff to care only for affected residents or only for unaffected ones, and grouping affected residents;
• restricting admissions, transfers, group activities and visits as directed;
• reinforcing hand hygiene, respiratory etiquette and masking;
• offering vaccines or antiviral medication where public health recommends it, for example antivirals during influenza outbreaks in long-term care;
• communicating clearly with staff, residents, families and partners;
• declaring the outbreak over only when public health agrees, usually after a set period with no new cases.

Outside health care, the employer's response to a cluster of respiratory illness might include reminding staff to stay home when sick, allowing remote work, checking ventilation, increasing cleaning and following any public health advice. Medical information about individual workers must be kept confidential.

Tuberculosis and measles at work

Two airborne diseases deserve special mention. Tuberculosis is uncommon in most of Canada and the United States but remains a concern in health care, correctional facilities, shelters, some Indigenous and northern communities and among people born in countries with high rates. Only people with active tuberculosis of the lungs or throat are infectious, and spread usually requires prolonged close contact in shared air. Many health care employers screen workers on hire and after exposures. Measles is one of the most contagious diseases known; up to nine in ten non-immune contacts become infected. The best protection is two documented doses of measles vaccine, and health care workers are expected to have proof of immunity. With measles circulating in parts of North America since 2024, workplaces such as clinics, emergency departments, schools and airports have had exposure events that require contact tracing and, for non-immune workers, exclusion from work during the incubation period.