Post-operative wound infections occur in about 7% of all surgeries and are the third most common healthcare-associated infection (HAI) in Sweden. For so-called “clean procedures,” meaning operations in bacteria-free tissue, the infection risk is 1–3% (Att förebygga vårdrelaterade infektioner, 2006). Most post-operative infections result from bacterial contamination of the tissue during surgery. Once the wound is closed, the infection risk is low. Bacteria can come from the patient themselves (endogenous infection) or from the surrounding environment (exogenous infection).

Endogenous Infection

In clean procedures, the patient’s own skin is the main source of endogenous bacteria. Most skin flora only cause infections in highly sensitive surgeries, such as implant procedures. increase particle dispersion. which is found in about 10% of healthy individuals and can cause infections in any type of surgery.

When operating on organs that naturally contain bacteria, such as the gastrointestinal tract, surrounding tissues are contaminated with large amounts of bacteria.

Exogenous Infection

Environmental contamination can reach the surgical wound through airborne particles or contact with instruments and fluids contaminated during the operation.

Skin is the main source of airborne bacteria in the operating room. A person walking releases approximately 10,000 skin particles per minute, about 10% of which carry bacteria. Particle size varies from 5 to 60 μm, with an average settling rate of 0.3 m/min (Noble, 1975). Anaerobic bacteria can also become airborne and survive long enough to pose an infection risk (Benediktsdóttir & Hambraeus, 1982).

Activity and friction, such as from clothing, increase particle dispersion. Clothing can fragment skin particles, and over 50% of bacteria-carrying particles may be smaller than 5 μm. People with skin conditions or infections can release large amounts of S. aureus and other pathogenic bacteria, e.g., group A streptococci. Ventilation and tight clothing alone are insufficient to prevent this risk. Infection control guidelines recommend limiting the presence of such individuals in the operating room.

Microbiological Air Requirements in Operating Rooms

Recent developments in surgical methods indicate that procedures involving the implantation of foreign materials will become more common, increasing the demand for sterile operating environments.

Guidelines align with the microbiological air cleanliness requirements in operating rooms described in Byggenskap och Vårdhygien 2010. These recommendations are based on older studies and what is practically achievable through ventilation combined with specialized surgical attire.

Ventilation in Operating Rooms

The operating room is ventilated to provide a safe and comfortable environment for both patients and staff. The guidelines mainly focus on aspects of ventilation that are important for preventing the spread of infections. The primary functions of ventilation are to maintain low levels of microorganisms during surgery, minimize the risk of microbial inflow from surrounding areas, and purify the air after the operation.

Airflow is expressed either as the number of air changes per hour or as the volume of supplied air per unit of time, usually measured in m³/s.

Airflow Standards (SIS TR 39)

To calculate the number of air changes, the airflow and the room volume must be known. When contamination spreads evenly in the room, the room volume does not affect the steady-state contamination level, which is determined by the total supplied airflow. However, the room volume does influence how quickly dilution and clearance occur.

The two most common ventilation principles in Swedish operating rooms are mixing flow with a supply airflow of approximately 0.6 m³/s, and so-called laminar flow ceilings with a total supply airflow greater than 2.5 m³/s. In mixing flow, the number of bacteria-carrying particles in the air is reduced through dilution. With laminar flow ceilings above the operating table, the intention is to remove bacteria-carrying particles from the surgical area. During surgery, however, laminar flow can be disturbed by the movements of the surgical team, heat generation, as well as the placement and heat output of surgical lights, causing the airflow to become partially mixing even in these systems. For both ventilation systems, the dilution principle can be used to calculate the required supply airflow (Nordenadler, 2010).

Preventing Microbial Contamination from Adjacent Areas

When a door to the operating room is opened, airflow can enter the room, posing a risk of bacteria-carrying particles entering due to temperature differences between adjacent areas and the operating room. Theoretical calculations show that this has little impact on air cleanliness in operating rooms with conventional mixing flow, where bacterial counts are often above 50 cfu/m³ (Nordenadler, 2010). A much larger source of contamination is when an additional person enters. The relative increase in airborne bacteria-carrying particles from opening the door becomes greater in operating rooms where very low contamination levels are required. To reduce the inflow of bacteria-carrying particles from areas directly adjacent to operating rooms used for infection-sensitive sterile surgery, these areas should have a defined air cleanliness level (Nordenadler, 2010).

A few diseases are spread through the air; these include chickenpox and measles. Patients with extensive burns release large amounts of bacteria-carrying particles into the air, contaminating the entire operating room environment. Hospitals with infection clinics and burn units should have an operating room accessible from a corridor outside the main operating area. To allow the room to be serviced with equipment, it should have an airlock entrance to the operating department.