A team of environmental psychologists investigates how biological allergens, such as airborne mould spores and pollen, act as environmental stressors. To study this, they conduct a field experiment in an apartment complex called 'Vanguard Heights', which houses 240 residents. They want to test the effect of installing medical-grade HEPA air purifiers in residents' bedrooms on their salivary cortisol levels (a biological marker of chronic stress).
Identify the independent variable in this study.
