Every year the smoke arrives, the sky goes orange, and most offices in this region respond the same way: an email telling people to stay indoors. That response treats smoke as a weather event. In California it is closer to a compliance event — and what determines which one it is for your building comes down to a few things a facilities programme either did or did not do before the season started.
Here is what actually applies, and what to do about it.
The number that matters is 151
Cal/OSHA section 5141.1, Protection from Wildfire Smoke, applies when the Air Quality Index for PM2.5 reaches 151 or above — the threshold the AQI scale labels “unhealthy.”
When it applies, an employer must monitor the AQI before and throughout each shift, tell employees what the current level is and what protections are available, train them on the health effects of smoke and on their right to seek medical treatment without retaliation, implement feasible engineering and work-practice controls, and make N95 respirators available for voluntary use. Above an AQI of 500, respirator use stops being voluntary and becomes mandatory, which brings fit testing and medical evaluation with it.
That is a substantial administrative load to spin up in the middle of a smoke event.
The exemption almost nobody reads properly
There is an exemption for indoor workplaces, and it is the most important paragraph in the standard for anyone running an office building.
The standard exempts workplaces “within enclosed buildings or structures in which the air is filtered by a mechanical ventilation system and the employer ensures that windows, doors, loading dock doors, and other openings are kept closed to minimize contamination by outdoor or unfiltered air.”
Read that again, because there are two conditions in it and both are physical.
One: the air has to actually be filtered. Cal/OSHA is specific about what that means in practice — filters should “fit snugly in their frames and have gaskets or sealants on all perimeter edges to ensure that air does not leak around the filters,” and the system does not do its job if filters are “dirty, damaged, dislodged or leaking around the edges.”
Two: the openings have to stay closed. Every propped fire door, every loading dock left open because it is easier, every window opened because someone is warm.
So the practical situation is this. A facility with clean, correctly seated filters and a closed envelope is in one regulatory position. The identical facility with a filter nobody has changed since spring and a dock door wedged open is in a different one — and the difference between them is maintenance, not budget.
None of this is legal advice. Confirm your own obligations with your safety adviser or counsel — but confirm them in August, not in the middle of an event.
What to do before the smoke arrives
Almost everything useful happens in advance. Once the AQI is at 180 your options are limited to whatever is already installed.
Find out what filters your system takes, and what it can take. Cal/OSHA notes most HVAC systems accommodate pleated medium-efficiency filters rated MERV 6 to 11, and some can take MERV 13 or higher. EPA recommends MERV 13 or the highest efficiency your system will accept. Your mechanical contractor can tell you what yours will handle without straining the fan — do not simply fit the highest number available.
Change them, then check the seal. A correctly rated filter sitting loose in its frame lets air bypass it entirely. Gaskets and sealed edges matter as much as the rating.
Stock spares. Filters load far faster during a smoke event than in normal conditions, and so does everyone else’s demand for them.
Walk the envelope. Doors that do not latch, dock seals that have failed, gaps around roll-up doors, propped-open stairwell doors. Fix them now.
Upgrade your entry matting. Track-in is a genuine route for particulate, and matting is the cheapest intervention on this list.
Decide who watches the AQI and where they will look. A named person and a bookmarked source beats an argument on the morning it matters.
What changes during an event
Set the system fan to “On”, not “Auto”. On “Auto” the fan only runs during heating or cooling calls, so filtration stops in between. EPA’s guidance is explicit on this and it is the single easiest adjustment available.
Close the outdoor air damper or switch to recirculate where your system allows it. Take advice before doing this — buildings still need ventilation, and this is a trade-off, not a default.
Keep the envelope closed and mean it. This is where facilities loses the argument with occupants every year.
Add portable air cleaners in the spaces people occupy most. Choose units with a published Clean Air Delivery Rate, and check they do not generate ozone — the California Air Resources Board publishes a list of certified devices.
Expect the filters to load early. Check them during the event, not on the usual schedule.
The part that cleaning gets wrong
This is where a cleaning programme either helps or actively makes things worse, and it is the part almost nobody thinks about.
Fine particulate that gets into a building settles overnight onto every horizontal surface — desks, sills, ledges, tops of partitions, light fixtures, the floor. The following evening, a cleaning crew arrives and, depending on method, either removes it or puts it straight back into the air.
EPA’s guidance on indoor air during smoke events is direct about this: avoid dry vacuuming unless the vacuum has a HEPA filter. The same logic applies to dry dusting and feather dusters, which do not remove fine particulate — they redistribute it.
What a cleaning programme should change during smoke season:
HEPA filtration on every vacuum, and the bag or canister changed more often than usual, because loading a filter reduces its efficiency.
Damp-wipe rather than dry-dust horizontal surfaces, with microfibre that traps rather than scatters.
Extend the reach upward. Ledges, tops of partitions, light fixtures and vents accumulate the most and get cleaned the least.
Increase entry and lobby frequency — that is where track-in concentrates.
Clean the mats themselves. A saturated mat stops trapping and starts contributing.
Do not add particulate. Aerosol sprays and fragranced products are the wrong instinct during an event, however much the building smells of smoke.
To be clear about the boundary: a cleaning contractor does not own your HVAC, your filters or your dampers. That is your mechanical contractor and your building engineer. What a cleaning contractor owns is method, equipment, frequency and reach — and those genuinely determine whether the particulate that got in stays down or goes back into the air every evening.
After the event
The most commonly skipped step is the one after the smoke clears, when everyone is relieved and moves on.
Change the filters again. They have done more work than a normal quarter.
Run a deep clean of horizontal and high surfaces before the settled load gets re-suspended by normal activity.
Return dampers and fan settings to normal.
Note what failed — the door that would not close, the filter size nobody had — and fix it before next season, because there will be a next season.
Frequently asked questions
At what AQI do California employers have to act on wildfire smoke?
Cal/OSHA section 5141.1 applies when the AQI for PM2.5 reaches 151 or above. At that point employers must monitor the AQI, communicate it to employees, provide training, apply feasible controls, and make N95 respirators available for voluntary use. Above AQI 500, respirator use becomes mandatory with fit testing and medical evaluation.
Are indoor office workers exempt from the wildfire smoke standard?
Only conditionally. The exemption applies to enclosed buildings where the air is filtered by a mechanical ventilation system and windows, doors, loading dock doors and other openings are kept closed. Cal/OSHA specifies that filters must fit snugly with sealed perimeter edges and must not be dirty, damaged, dislodged or leaking — so a neglected filter or a propped-open door can put the exemption in question.
What MERV filter should an office use for wildfire smoke?
EPA recommends MERV 13 or the highest efficiency the system can handle. Cal/OSHA notes most HVAC systems accommodate MERV 6 to 11 and some can take MERV 13 or higher, with low-pressure HEPA (MERV 17+) worth considering where smoke exposure is frequent or occupants are vulnerable. Check with your mechanical contractor before upgrading — a filter too restrictive for the fan causes its own problems.
Does cleaning help with wildfire smoke indoors?
It helps if the method is right and hurts if it is not. Fine particulate settles on horizontal surfaces overnight, and dry dusting or vacuuming without HEPA filtration puts it back into the air. During smoke season, HEPA-filtered vacuums, damp-wiping instead of dry-dusting, more attention to high and horizontal surfaces, and more frequent entry-area cleaning all make a measurable difference to what stays airborne.
Get your building looked at before the next event
JAN-PRO Commercial Cleaning in Sacramento works with offices, medical facilities and industrial sites across Sacramento, West Sacramento, Roseville, Folsom, Elk Grove and Rancho Cordova. Our free facility visit takes about twenty minutes: we walk your entries, floors, horizontal surfaces and high areas, and tell you what your current cleaning programme is and is not doing about settled particulate.
We will also tell you plainly which parts of this belong to your mechanical contractor rather than to us.
Call (916) 376-8977 or request a facility visit online.